Its possible to backstab front facing players by priming your butterfly knife on front facing players and then changing your target to your victim.
This YouTube is the best example of how to preform the exploit. Its all about timing.
At first glance you would say that this is a critical knife to the front. Please notice the icon in the upper right corner that clearly marks the kill as a backstab.
A more advanced version of this 'facestab' can be done by using movement to prime your backstab on the same player you kill.
How To Backstab In Tf2
How To Backstab People From Far Away In Tf2
One of the big problems in TF2 right now is 'facestabbing' where a spy will seemingly be in front of you, or even running right at you, but miraculously land a backstab one hit kill on you. A thread posted in the Steam forums has a brief explanation as to why this can happen: The. Games Team Fortress 2. Follow/Fav Backstab. By: ForeverYoung0422. Jen is the only girl on the team so needless to say it was going to be hard for her. The only difference is in the animation. Something you do the swipe, something you do the backstab motion, but it all takes the same time. Valve also added a thing where if you are in a backstab position, before attacking at all, you will raise your arm. This is purely a visual helper and doesn't affect the result of your backstab.