Well, I must admit the thread title caught my attention. I even said "Okay" out loud.
What you want to work on is minimizing the possible time for somebody to spot you, and that means only two things. You must find ways of getting higher the the opponent you are going to stab, or you need a REALLY kickass hiding spot.
On well, for example, you can do both. On arena well, especially (people don't rush the center as fast), here is an example of a kickass hiding spot:
I can almost guarantee you that you will get surprisingly easy kills here. You must understand why it works; When people run in, they get an easy full-area view, so suddenly the comfort level rises a lot. Just look at everything they see:
And the one thing they don't:
If there is anybody else on your team, they will be looking at them, as they proceed up the what they thought was safe ramp area, and bam. Easiest way to pick off medics in the world, even get combos sometimes because it's like "wtf is this. Now, don't get me wrong, all it takes is a glance into the corner and you're done. You won't be catching me off gaurd anytime soon, but the reason this spot is so much more effective than other corners is that the player has so much to take in when they enter the area, but as usual don't rely on it. But try it out, and get a feel for it.
Next and last is height. When people are spy-scared, they'll turn around constantly, looking everywhere. These guys cannot be reliably stabbed if you're on the same height level. But, if you get higher, even jsut a player's height above them, they won't see you. When they turn around, the look at exactly what's around them. Next time you get spy-aware and start turning around, try and take a moment to notice where all your attention is: not up. You can get in the habit of it (watching me look around is like watching a dog try to catch his tail), but this is by far the most reliable way to kill somebody.
OMFGNinja calls it a leap stab which I must admit I tihnk is the most retarded name you could come up for it. If anything, it's a fall stab, as a leap implies some sudden lurch in forward momentum, where there is not. And while we are here, it's flick stab, you FLICK the mouse, not quick stab. A quick stab implies doing something faster than was intended. As such, a quick stab would refer to, say, not getting the lift when you normally would have.
Back on topic,
here are some old clips of height advantage, editted together really fast. And on a side note, to stab anything below you, you need to crouch to lower yourself. The way you backstab when you're on a different height plane can be tricky. Try imagining it like this.
Take a sphere. This is the enemy. This sphere has a direction, and it's also split in half. That's the front and back, and you can imagine the back half of the shpere like a shell on the player (TEENAGE MUTANT NINJA TURTLES). Now you need to imagine the plane that cuts that sphere in half. Got it?
As the player rotates, the plane rotates. Here's how it works: if you hit the player and you are behind this plane, he will get backstabbed. Now you see that backstabbing depends on your position relative to the player and his rotation. But your rotation has nothing to do with it. If you could somehow hit the player without turning, you could hit him while facing away from him and bkacstab him. (You can get close though, if you flick stab 180 degrees).
So when it comes to stabbing from the top, do not worry about which way you are facing. Rather, just try to poke him while you are behind him. To make matters simpler, look straight down and then just be behind when it hits. It'll take a bit of practice.
Cool thing is, if he turns around and it only swipes him, you're probably gonna be standing on top of him in an akward situation, where now you can just try again.
So work on:
1) Creative hiding spots (know what they are thinking/will do), which automatically includes:
2) Height advantage.
3) And in addition, having really good revolver aim is tops.
So yea, I hope that answers your questions.
Here's the link again for anybody that doesn't read all my post:
Video.