As he attempts to hurdle the beam, you could compel his slash to the calf consequence to make the attempt harder.
That's a valid use, but it's not a compel. It's invoking your opponent's aspect for your roll. (Or for the fixed difficulty.)
A compel would be preventing the leap in the first place...
Player: "I'm going to leap the beam"
GM: holds out fate chip, "That slash in your leg seems to indicate you'll not be able to leap"
If the player accepts, he's not supposed to leap at all; if he rejects, he hands over a fate chip and makes the leap at no difficulty.
Quoting from the "Fate Basics" PDF:
Invocations -- Spend a FP to Invoke an Aspect. The Aspect may belong to the acting character, the defending character, the scene, or an object involved in the action.
Invocations have one of two results. The active player gets +2 to the roll or may reroll the dice and keep the new result. FPs are spent after the roll is made, and each Aspect may be Invoked only once per action.
On unrolled actions, Invocations simply add +2 to the Skill’s rating.
When Invoking an Aspect belonging to another character, any FPs go to the player of that character once the action resolves.
Note the ability to invoke an opponent's aspect.
Declarations -- The player spends a FP to make some narrative detail related to the Aspect a fact. The GM has the final say on what is and isn’t allowed, but players should be able to make moderate alterations to the story this way.
Declarations do not involve any sort of roll.
Compels -- The third thing about Aspects is that they can be disadvantageous. This is in the form of a Compel. Compels dictate some constraint to behaviour.
When one is made, the GM offers a FP. If you accept it, you accept the constraints. If not, you must turn down the offered FP and spend one of your own.
Note that Compels don't involve die rolls at all.
It's also worth noting that many Fate system games have consequences as a limited form of aspect - the player can't invoke them, but everyone else can invoke or compel them, or declare based upon them.
At the most basic -
— A compel is a "you can't do that, here's a fate point."
— An invoke is "+2 for one side by paying a fate point to the other side" and can be in either direction, but is done AFTER rolling.
Also note there is a difference in the ending fate point total for negations...
If you compel, the player will end with either +1 or -1 fate points - depending on whether he accepts or rejects the compel.
If you invoke, there are three possible outcomes - accepted, accepts but negates with another aspect, or rejects. Accept and fail gives them +1 fate points; negate with other aspect comes out neutral - invoking the second aspect costs only 1 fate point, effectively canceling the invoke. Rejecting the invoke outright costing 1 fate, for a final total down by 1.
Also note the sequencing - Invoke is AFTER rolls, while compel is before.
To give an example of how it works as an invoke...
Player: "I'm going to jump that flaming beam"
GM: "Roll - it's Good difficulty"
Player: rolls. "Made it with a Great result"
GM: holds out fate chip. "That slash in your leg hurt a lot, and you didnt' get as far as you thought - only a fair result"
Player Response 1: "Ok, I failed." Takes the fate point"
Player response 2: takes fate point, the hands it back, saying "Sure, it hurts. But my 'boxer's constitution' lets me tough it out."
Player response 3: picks up one of his own, instead, and tosses it to the GM. "It may hurt, but not enough to impede me from making it over."