I went in and practiced this the other night. I'm able to get the hand in the face contest to fire off pretty consistently, but you are right, its tough to get it to fire "every single time".
You have to be in perfect "locked-in close" (you know how there are three locked on distances?), and its damn hard to get into that position on the fly.
Like Born mentioned, it can be done regularly, but the system is VERY picky with the spacing.