I read Tome of Mordenkainen's enemies when I stumbled on an interesting turn of phrase (it is repeated often in the book and in others, but this is where I first became aware).
The Demonic Boons section lists the following trait for followers of Graz & # 39; zt (p. 30):
Joy of pain. Whenever this creature is critically hit, it may attack with a melee weapon. in reaction.
Meanwhile, Xanathar's guide (p.46), the misleading sub-class feature of Mastermind thief says:
When you are targeted by an attack while a creature within 5 feet of you protects against that attack, you can use your reaction that the attack targets this creature instead of you.
Is there a mechanical difference between these two sentences? If something is done "in reaction", is it not "use your reaction"? Or are they simply interchangeable sentences?