Duckbug
Duckbug is a technique to avoid fall damage. The big advantage of the technique is that it's about two times easier than a jumpbug, but unlike the jumpbug it can't be performed everywhere - you'll need a slanted surface to be able to do a duckbug.
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
It's possible to trick the engine if you are ducked in the last airframe and release duck when you touch the ground. However, the cs engine won't detect that, because you gathered a lot of speed while falling down causing that the next frame will be in air again, making the engine think that you didn't touch the ground at all. Nevertheless your vertical speed (falling speed) is reseted, just like if you did an edgebug, therefore you won't die anymore.
How-to
First of all you need to gather some speed (more speed than just running off a high block and falling down) and jump or run off a high block. The less slanted the target surface is the more speed is needed - and the more slanted the target surface is the less speed is needed for a duckbug.
Then, while you are in the air you need to hold duck and release duck (unduck) being very close to the ground in order to trick the engine.