Arg TRY IT ! There is an animation for it ! Just get close to the zombie with a knife and STAB ! You're right, I've seen it, but it has to be really close xD It's hard to miss it because, either I'm too far away and miss, or I get to near and the zombie spots me. And even so, it doesn't work every time. I managed to do it maybe 5 times, in over 70 kills in various playthroughs, but it's there. Sorry for the "not so" suggestion xD