Simple. A d6 is rolled, and if a 1 is the result...That is not generally something you, the gm, should be doing though. The players cannot fail forward and the story has nowhere to go if npcs are just executing them mid fight.
Speaking of Quickdraw, would you allow a player to draw and parry on an enemy's turn?Nope, that's what quickdraw is for =)
Although, generaly if they just want to take a couple of steps I'll let them do that as a free action.