In the games I played, the GM didn't use it much, he didn't like the extra tracking. Me, I don't mind.

Problem is, outside of how one gets knocked down, I cant find much on it. Anyone know where the rules for it are? Im wondering if it effects movement, penalties for being shot at, or shooting, if it's an action to get back up, etc.

Stand Up is a Simple action (if wounded, requires a test). There's a -2 penalty to defense (melee, and ranged <5m away). I think crawling is impossible by RAW, heh.

It's a simple action to stand up. Needs a Body + Wil [2] test if wounded.

Being prone nets you a -2 on defense rolls both for ranged (only if attacker is within 5 metres) and melee. A melee attacker also gains 3 extra dice on prone people.

And using Quick Reply incurs a -20 modifier on defense against being nijaed.

"Nijaed"?

Thanks. Any idea on page in SR4a?

Seams, ah cand speel twoday.

Well, I only got the non-Anniversary edition, so no page numbers, but chapters:

Stand Up: Combat / The Action Phase / Simple Actions
Opponent Prone: Combat / Melee Combat / Melee Modifiers Table
Defender Prone: Combat / Other Combat Factors / Defense Modifiers Table

HTH