Quantum Crouch
Overview
Quantum Crouch (QC) is a glitch that allows you to keep the standing vertical view offset while crouching. Because crouching hitbox is 36 units tall and standing vertical offset is 64 units, your camera is 28 units above the top of your collision box, allowing you to put it in places not easily accessible for player.
Execution
To get a QC, you need to have two portals with different orientations (most commonly, axis-aligned floor and wall portals are used). Then, walk into a floor portal so that you're next to the bottom edge, and hug to it using a directional key. As soon as you fall into a floor portal, you should pop right back from it. When that happens, immediately press and hold a duck button and try to strafe away from a portal. At this point you should have QC. To check if that's true, your view offset should be high (it should look like you're standing) but your movement should be slow.
One thing worth mentioning is that sometime you can get something called half-QC. This is caused by a flag that's telling the game whether you're crouching midair or on ground. This flag makes the view offset for crouching/uncrouching different because of the difference between ground crouching and midair crouching. To avoid that, make sure to tap a crouch button before attempting to get QC to reset that flag, so the game thinks you're crouching on ground.
Usage
TBW
Technical Explanation
TBW