Fire at the arms of a disarmed enemy Ai Unit until that Ai unit is wounded (But not dead or incapacitated).
Get close and if you have a mediKit or a first aid kit you should see the default action appear. It gets completely in the way of my custom melee "take down" addction that appears on enemies when you get close to them. (And that is just one of the many examples of why some missions might require to disable that or any other particular default action.