Vampire spawn have the trait "chained to the grave" which includes the phase "Every vampire remains bound to its coffin, crypt, or grave site, where it must rest by day."
Vampire spawn also have the flaw, "Stake to the Heart. The vampire is destroyed if a piercing weapon made of wood is driven into its heart while it is incapacitated in its resting place." True vampires are merely paralyzed by such an attack, and no mention is made of them being incapacitated while resting.
What does "must rest" by day mean? Are vampire spawn not allowed to be out of their grave sites in the day? Could they choose to be up and about (subject to the rules of damage in sunlight) with the only penalty being that they are forgoing rest, or are they rules-required to be in their grave sites? Can they break their rest and wake up if damaged during the day, or if they hear something with Passive Perception per the optional rules for sleep?
[Note that while this question is similar to What happens if a vampire can't rest in his coffin?], it is not the same, in that it specifically asks about vampire spawn. The higher voted of the two responses in the other question references Mordenkainen's Tome of Foes, saying that vampires that cannot rest become vampiric mist. But I am asking about vampire spawn, which do not have access to a true vampire's misty step ability.]
Does the state of "rest" that vampire spawn are in also incapacitate them, in the same way that an unconscious PC is incapacitated? Or must characters find some way independent of it being day to incapacitate them in order for the stake to instantly kill them? That is, is the incapacitation a result of the forced rest in the day, or an independent requirement in order to destroy them?