TF2 Spy Technique: Overstab
A lot of people call this stairstab. I didn't know about the name until after I had known and used the technique for some time, so I've always called them overstabs. Ah well.
The overstab is a technique that requires an oncoming, downhill target. It involves a jump+crouch over the target, quickly followed by a backstab. A vast majority of foes generally won't be ready for it, and with practice, it works most of the time; even practiced players generally won't be able to turn around in time unless they specifically know you're about to use that (which can happen on repeat overstabs). Interestingly enough, in my experience, it seems to work best on pyros.
This is also my second attempt at a youtube upload. The first one failed to format correctly. Let's hope this one works.
~G-Mang