A stucklaunch is a trick that utilizes continous jumping in a single spot for gaining high amounts of velocity in a short amount of time. It is one of, if not the fastest way to gain thousands of units of speed from a standstill.
Despite being possible in both Old Engine and New Engine, in OE Stucklaunching is generally not used in favour of Pauseboosting
A stucklaunch is the result of multiple accelerated hops (In New Engine) or multiple bunnyhops (In Old Engine) ontop a physics object, performed while "stuck" in place. If you continue to jump while stuck, your speed will continue increasing like a normal ahop or bhop, but with the benefit of not spending time in the air, so you can immedielty hop again.
There are multiple ways to perform a stucklaunch.
The first, and easiest way is to simply jump on top of a slightly upwards tiled physics object. Some are better than others, with brush entities being particularly great.
The second way involves a usespam script, which rapidly picks up and drops pickupable physics objects.
By usespamming, while traveling forwards and having the prop underneath the player, they often won't have time to jump before another regrab happens, allowing them to become "stuck" in place temporarily while jumping.
In New Engine the player is required to already have speed when initiating a stucklaunch, due to the way ahopping works.
It is possible to do it forwards, either by holding +strafe or s, and backwards, without holding any additional keys.
It is not required to already have speed in Old Engine, Holding w when launching forwards, or S while launching backwards is all that's necessary.
It is also possible to use the gravity gun to punt the object you are standing on to get launched into the air and gain signifigant height.
TODO: add examples for launching with usespam, and maybe OE, though that would be pretty pointless.
ExtraVirus' stucklaunch example: https://youtu.be/eKqjNWdvNmg
Maxam performing a forwards stucklaunch using +strafe: https://www.twitch.tv/videos/948105876?t=00h10m29s
Link to old wiki page: https://wiki.sourceruns.org/wiki/Stucklaunching