How does the Open Hand monk's ability to prevent a target hit by Flurry of Blows from taking reactions work?

Question

The Way of the Open Hand monk's Open Hand Technique states:

Starting when you choose this tradition at 3rd level, you can manipulate your enemy’s ki when you harness your own. Whenever you hit a creature with one of the attacks granted by your Flurry of Blows, you can impose one of the following effects on that target:

• It must succeed on a Dexterity saving throw or be knocked prone.
• It must make a Strength saving throw. If it fails, you can push it up to 15 feet away from you.
• It can’t take reactions until the end of your next turn.

How does this work with regard to spells such as Shield or Hellish Rebuke? Are they just canceled?

Answer 1

This should be made clear by the "Adjudicating Reaction Timing" section in the Dungeon Master's Guide, page 252.

...sometimes the timing of a reaction be difficult to adjudicate. Use this rule of thumb: follow whatever timing is specified in the reaction's description. For example, the opportunity attack and the shield spell are clear about the fact that they can interrupt their triggers. If a reaction has no timing specified, or the timing is unclear, the reaction occurs after its trigger finishes, as in the Ready action.

The Flurry effect is not a reaction, it happens upon the hit landing. So the general rule here would be that the flurry hits, all the effects of that hit resolve, then any reactions to that hit resolve; but shield is called out as a specific exception to that.

Shield precedes the attack that triggered it, potentially turning a hit into a miss and canceling its own trigger condition. Given that, and the fact that the Flurry effect can only happen upon an actual hit, the shield takes precedence in this scenario. If you throw a Flurry of Blows at somebody, they can cast shield before the first one hits, gaining +5 AC against the entire flurry and potentially preventing hits. If one of the attacks hits anyway, you can choose to use the "no reactions" effect, but it wouldn't stop the shield from working. The reaction already happened, now it's just a game effect that's already in place.

The target regains their reaction at the start of their turn, but your punch could prevent them from using that reaction until the end of your next turn, which could potentially prevent them from using another shield against your attacks on your next turn.

By contrast, in the case of a hellish rebuke, there is no specified timing, so the general rule applies. The monk's attack hits, the effects of that attack resolve, and by the time the reaction could theoretically happen, the target is incapable of using it.

Answer 2

In these specific cases, the rules are explicit.

Open Hand happens before hellish rebuke.

Since hellish rebuke triggers when you are damaged and Open Hand triggers when you are hit before any damage is dealt, Open Hand unambiguously triggers first.

Shield happens before Open Hand.

See Darth Pseudonym's answer for the passage in the rules that reinforces that shield is intended to occur first. This passage isn't in the text of the spell, which is problematic, but it is there in the rules and does resolve the ambiguity. I won't replicate Darth's answer here.

In the general case, there is an optional rule.

Notably, hellish rebuke and shield aren't the only ambiguous situations that might ever come up when using Open Hand, and there is no core rule covering them all, but, the problem of simultaneous timings comes up often enough that an optional rule was released in Xanathar's Guide to Everything, in the aptly named section on Simultaneous Effects:

If two or more things happen at the same time on a character or monster’s turn, the person at the game table — whether player or DM — who controls that creature decides the order in which those things happen.

If you use this optional rule consistently, then the player of the monk (whose turn it must be, since they just used Flurry of Blows with a bonus action) can decide whether its Open Hand effect triggers first or whether its target's shield spell triggers first.

It's likely the monk will choose to trigger its effect first (in which case the target no longer has their reaction to use in order to follow the hit up with casting shield), unless it wants to compel the target to spend a spell slot (in which case the shield spell prevents the hit from occurring and the monk still tactically benefits from the result). Regardless, the decision would be up to the player of the monk.

Using this optional rule fits nicely with a DM's intuition that Open Hand is more specific than shield and that the monk should be allowed the chance to shine in this way. It's also pretty clear that the intention of Open Hand is to provide a way to rob a target of a reaction, and this ruling elevates that intent. As a bonus, this ruling also works for shocking grasp and other features with similar timing.

This ruling violates the stated intent for shield in the rules, as explained in the previous section, but my goal is to teach you to fish rather than giving you a particular fish. Ideally, knowing about this optional ruling will help you resolve future issues of ambiguity with Open Hand.

Answer 3

Shield still works. Other reactions don't.

The timing of shield is squirrelly, in that as written, it can negate its own triggering event. You're "hit" by an attack, you react by casting the spell, and this raises your AC against the attack, which can make it not hit you. If it doesn't, then none of the effects of being hit apply, except for being able to cast shield.

The only non-paradoxical interpretation of the spell is that at the time you can cast it, the attack roll is not completely resolved (because your AC against that attack still matters). It says "when you are hit by an attack" but it really means "when an attack roll is made against you, and would hit". You are going to be hit, unless you cast some kind of very fast spell to save yourself.

(This interpretation is supported by the spell's effect, which is to create a "barrier of magical force", represented as a flat bonus to AC. It's not reversing time so that the attack never happened. It's providing a fancy kind of partial cover.)

Take a simpler case:

DM: The troll swings a treetrunk at you. (Rolls a 12.) That will hit AC 18, is that a hit?

You: Yes. I cast Shield.

DM: Okay, but you got hit, so (rolls some dice) you take 21 damage. How many HP do you have?

You: I have 19, which is why I cast Shield.

DM: Well now you have zero, you fall unconscious, and you can't cast Shield.

This is clearly incorrect, and the rules treat special effects like this Monk ability as having the same timing as damage from the attack:

On a hit, you roll damage, unless the particular attack has rules that specify otherwise. Some attacks cause special effects in addition to or instead of damage.

Applying the rule from Xanathar's is problematic.

The general problem with tiebreaker rules is getting everyone to agree that there is a tie to be broken.

The simultaneous effects rule applies only when they really are simultaneous. In this case I don't think they are, and applying that rule is clearly begging the question.

Answer 4

Reactions

Some spells can be cast as reactions. These spells take a fraction of a second to bring about and are cast in response to some event. If a spell can be cast as a reaction, the spell description tells you exactly when you can do so.

"... can't take reactions ..." means "... can't take reactions ..." so, you can't cast a spell that needs a reaction to cast.

Both Way of the Open Hand and Shield have the same trigger "when you hit/are hit" - so which happens first?

Option 1

The Monk's Way of the Open Hand, being the more specific (a particular choice of a particular class feature) beats the Shield (being a spell that works against all hits).

Option 2

Follow the optional rule in Xanathar's Guide to Everything:

In rare cases, effects can happen at the same time, especially at the start or end of a creature's turn. If two or more things happen at the same time on a character or monster's turn, the person at the game table - whether player or DM - who controls that creature decides the order in which those things happen.

Since Flurry of Blows is a bonus action it can only happen on the Monk's turn - barring some very unusual edge case (involving wanting the enemy to use up spell slots/reactions?), the Monk would normally choose for their effect to fire first.

If you had previously cast Shield this would not end it but it does prevent you from casting it.

Hellish Rebuke is easier - it triggers when you take damage which happens after you are hit.