The Spy can be a powerful asset in the hands of someone who is familiar with the game engine and how the game handles hitboxes. But just because someone knows how the stairstab works it doesn't mean they're infallable - if they can't remain unpredictable and dangerous they're no better than a trained monkey.

A trickstab is the term used for taking advantage of the games very generous backstab hitboxes. Look at the diagram, the red circle is you and the line going through you represents where your backstab hitboxes begin and end. It is this hitbox that the Spy needs to aim for by baiting his victim into either chasing or meleeing him.

There are various ways this can be done and there are as many ways for ruining their fun. I'll list each possible attack and provide a counter-strategy (and you don't even need to switch to Pyro!)

Sounds cool? OK!

I don't feel this guide will be OTT enough without coining a silly phrase of my own, and that will be 'Oh No You Don't' (ONYD) range. I'm going to be using this term a lot in this guide because if fighting one of these guys hasn't already taught you not to give them a hug, you need it spelled out.

If you see the spy or you see through his disguise and he isn't already fleeing, it means he wants to bait you into exposing your hitboxes. If that's the case don't oblige him and melee him (1), this is the range the trickstab Spy wants. Instead strafe and use a close-range attack weapon (2). This tells him that you're aware of his plot and he can either flee and try again later, set you up for another trickstab or die an idiot's death.

The reason I'm putting this in it's own section is because this is the golden rule: STAY AT ONYD RANGE AT ALL TIMES WHEN BATTLING A TRICKSTABBER! No exceptions. As I said above; the trickstabber is working on the basis that you'll try and melee him or even get near him for long enough for him to exploit your own position.

These are the most common stabs you will encounter, and ONYD range applies to all of them.

What it is: The infamous stairstab, so-called the stairstab because when you're chasing a Spy up some stairs or a ramp he will leap over your head and score a cheap backstab. Don't forget that this can also apply to ANY elevation advantage, so if a part of the map goes up slightly or the spy is sitting on a dispenser, the same thing will happen.
Counter it by: Follow the Spy up the damn stairs (it's what he wants) but at ONYD range. As soon as you see those feet leave the floor MOVE BACK and aim at him. Got it? BACK UP BUT AIM AT THE SPY! Nine times out of ten he will land right in front of you and will look very stupid... You on the other hand will still be in ONYD range. He will not have a hope in hell in retreating if you're playing a high-damage class or if your team happens to be around.

What it is: Again, you're persuing a Spy and you go to turn a corner. Suddenly, backstabs. Why? Because when you're turning the position of your hitboxes are moving around with you, the Spy knows this and simply telegraphed when and how you'd be taking the corner.
Counter it by: Strafe along the corner or bait the spy into moving in for the cornerstab and back up to ONYD range. Either he'll keep trying and risk drawing more attention to himself or leave you alone.

What it is: Two people walk past each other, one a Spy and the other his victim. The Spy quickly turns around and quickly backstabs the victim. Simple, but that's why it is so called the quickstab - it's quick.
Counter it by: Most players on your team will either deliberately or unknowingly walk through you on several occassions. A spy doing the same thing wouldn't work so they walk slightly off center to avoid bumping into you. If you see a friendly player moving your way but not intentionally walking through you just be on the safe side and spycheck with a ranged weapon. If they look at you or try to avoid the interaction, they're most likely a spy. Spycheck them at ONYD range and if they flee up some stairs or round a corner then they are trying to bait you into the above two trickstabs respectively (see above).

What it is: A spy with the knowledge on how to pull this trickstab is dangerous and the entire reason I am coining ONYD range as my own. The facestab or matador stab is all about exploiting that hitbox in a melee fight and close range.
Counter it by: A Spy that has been confronted and knows how to pull one of these off is not someone you want to melee. But here's the trick: He wants YOU to make the first move - always. When you run towards him he is going to turn back around and hit your back. If you deny him the satisfaction of not wanting to go first and ONYD, what's he going to do? Move closer? No of course not. Don't move in close to him, don't chase him up stairs or around corners and just keep lay down hits at constant ONYD range.

As for the other stabs like the leaping stab or the understab these trcikstabs work on using the map geometry more than exploiting hitboxes so there's not much you can do except remain alert, listen to footfalls and fall damaging players, stuff like that.

If this has helped you please pass it on. Knowledge is power and it is better for everyone to build up a resistance to trickstabs. A trickstabber confronted with someone armed with foresight or even the slightest idea of what he's up to is gonna have to work harder at the very least.

Again, pass it on, spread the word. Thank you for reading.