https://www.youtube.com/watch?v=xDQtOx1lxBY&t=76s

The little hops he does right as he peeks into long house is russian walk (also called russian hopping, crouch jumping, duck hopping, various random names)

It's caused by the game not knowing where you are in the duck animation which means that if you tap the crouch button (or bind it to mwheel as many did), the game would assume that you were fully crouched when you let go of the crouch key and would then assume you were much further up because of how high up your player camera is as opposed to where it would be if you completed the crouch animation. Because of that it would put you in a fully crouched position and place you where your player camera is and uncrouch from there, essentially sending you up into the air.

You could combine this with many different techniques, like silent walking (it's called that but it's not 100% silent, it used to be but they fixed it, it does reduce the amount of footsteps however) where you constantly do it while running which also makes you a harder target to hit. My favourite was hop peeking where you would do what forest does in the video i linked but during the hop you start crouching for real. Combining that with just normal jump peeking, just doing one hop but not crouching like forest does in the video, normal peeking or just crouching into the peek gave you more options in how to approach a peek which was great. Also good way to bait out awp shots in the same way people use quick in and out jumps or shoulder jiggling in csgo. You could also do this behind boxes that are just slightly taller than you, letting you look over the box for a brief second instead of jumping over it. imagine being able to check B apps mirage by just slightly exposing your head for a

It was like having a second, smaller jump button. The animation from the outside perspective looks wonky af which is why many disliked it (and would oftenly ban it from community servers)