When a player attempts to use skill Aright at the moment it is switched with skill B (i.e. usually weapon swapping, but the same applies for getting downed), skill B will be cast instantly instead. Also, if skill Bwould have ground targeting (with fast-cast off), it is cast instantly regardless, following these rules:
If the player has a hostile target, the skill will be aimed at that.
Else the skill is casted at the player’s position exactly.
Easy reproductions:
Thief: use D/D and shortbow. Press #5 (while having D/D equipped) and 0 (=weapon switch key) with the right timing; you’ll cast Infiltrator’s Arrow on your location (or the enemy’s) instead of C&D.