First time watching the video I didn't catch how to do the Poosting, but now I get it! To anyone else struggling, you can start by watching Ansy's video tutorial.

Spear weapons from melee category "Light Poke" on single attack click will make you dash slightly forward then backward (hence the name "PooSting", letters from Boosting replaced for SPear letters). These vary in range (attack range and poost distance), damage and attack cooldown, but for speedrunning Range will be most important stat, which Gatehouse Spear (Gatehouser) happens to have highest +5. It's also most expensive one, so take a notice while buying not to pick similarly looking Gatehouse "Heavy Poke". The Heavy Poke boost needs a charge that greatly slows you down, and while boosting/dashing you can't change your direction. No idea if it's faster than "180 poosting" with Light Gatehouser, if you make a comparison video please share. In a speedrun environment, it's unlikely it'd be faster.

This makes for several Light Poosting techniques:

1. Bow-switch Poosting requires you to switch to bow to cancel that backwards dash. Since around [2023-08] some update reduced Switch-poost efficiency, where backwards dash can't be fully canceled.
2. 180deg Poosting uses that same forward dash, but then you turn back to catch a wind of second dash.
3. Wall Poosting uses the fact forward dash recoils off collision bodies, so just face a wall and fly backwards.
4. Ladder Poosting is used to jump unexpectedly high off the ladder's top. At its core it relies on Edge Poosting, but on difficulty increased by ladder physics. Reach the top of the ladder (where there is that weird catching when going up), then aim downwards and a bit to the ladder, finally jump and boost at the same time. Some ladders are long and you with the climbing process was quicker, luckily all other poost techs work on ladders.
5. Edge Poosting is when you poost into the edge of a prop to get an extra high jump, preferably that edge is smoothed. Essentially the angle you look down should be 20~40°Poost directly into the object without jumping (much more reliable), while pressing forwards, so that you lunge into it. Then press back while jumping with the recoil. It was previously thought to be based on RNG, but it seems to be based on hitbox interactions. ... ... [2024-02-02]: Below methods 6, 7, & 8 - have been patched to not work anymore.
6. Omnipoosting as name implies allows poosting in any direction - lock your head in the only angle that you don't intend to poost (look straight at the floor), then switch to 3rd person camera ("Y" key) and Poost to move around. Keep in mind that for Poosting techs that require you to look straight up/down you just press Attack to poost, no switch or aggressive mouse movement and you gotta time the clicks with player animation. If you run out of dashes you switch the weapon (Press "Q" twice) to reload. This tech allows long difficult jumps where you can rely on speedhack anticheat, that stops you mid-air when exceeding speed limit.
7. Crab Poosting also has you look straight up/down, but in first person camera. Then you gotta jump sideways. Since [2023-10-03] patch, jumping sideways isn't a requirement, you can Crab Poost forward or backward as well. Since then you'll see people call it Floor Poosting.
8. Floor Poosting starts by looking in the direction you want to go then lunge, then jump, then look straight down and hold back till the animation finishes, then hold forward again and repeat. Before Crab Poosting had its improvement patch, it was a more convoluted way of bypassing the "jumping sideways" requirement.

Credits: "1)" by 12bobs [2023-01-14]; Video by Toafloast [2023-03-21]; "4)" by Creepy [2023-05-20]; "6)" by gm.razgriz [2023-08-09]; "5), 7), 8)" by lege19 [2023-09-09]; "5)" description by mewcopult [2023-10-10]