Slidebug
Slidebug is a technique to avoid fall damage. This technique is similar to the edgebug, but obviously - just like the name is implying - we need a surfable or slideable wall to perform this, not an edge.
Another difference compared to the jumpbug is that you won't lose your horizontal speed, so you should prepare to slow the "fall" down after you performed the duckbug.
Explanation
The result of this move is that we are touching the ground and sliding up a wall at the same time, forcing the GoldSrc engine to assume that we have a positive vertical speed (going up) and that we are are not connected to the ground - so there is no "need" for the engine to damage you.
How-to
First of all we need to find an object we can slide/surf on, for example a surf wall or a spike (just like the edges when we try to do an edgebug). Running and jumping towards that object has to get us up 64 units high, otherwise a slidebug is not possible on that object. Furthermore this object needs to be touching the ground obviously, so we can stand on the ground after performing that slidebug.
After we found an adequate place to jump off from we need to make sure that we maintain enough horizontal speed to be able to touch the ground and - exactly in the same frame - touch the wall of the shortly after we were landing. After we "touched" the ground (which would make us die) we need to strafe into the slanted surface of the surf/slideable object immediately. If you do this with a perfect timing you won't die and survive the fall.