I saw a video (specifically this: link).

And wondered what a viewsnap is and how to do it.

a viewsnap is setting in-game sensitivity to an extremely high and precise value to perfectly aim a shot or angle, to do a strat with one only really requires knowing the sensitivity for the given route and how to set up the starting angle (typically entering a floor portal, since your yaw gets reset)

with the sst plugin used in runs (see resources) pretty much everyone sets sst_mouse_factor 20 to emulate lowering their mouse dpi so its a little easier

How do you do one?

Viewsnaps are highly situational; how you do one depends entirely on what you're trying to do.

In general, if you need to move your camera from angle A to angle B, you first find the difference (i.e. -90 to -43.24 = 46.76) and then divide it by the value of m_yaw, which should always be 0.022 (i.e. 46.76 / 0.022 = 2125.4545)

By setting your sensitivity to that value, your view will move in increments of 46.76 degrees.

This is only useful if you can first find a way to get a consistent viewing angle and position. Again, highly situational.