You have to be just in front your enemy to a whip distance and press B.
The easiest chapter to get this is in the last one of the last crusade.
There are 2 trucks where enemies keep getting out while you don't close the back door of them.
Just disarm them kill them and new 6 enemies will appear.
Just and advice if only 2 enemies are left your teammate will keep going from one to the other stunning them so you won't be able to disarm them. Just kill one and your team mate won't do anything to them.