Rules Compendium p92 (and Complete Adventurer page 101)
Hide (Dex; Armor Check Penalty)
You need cover or concealment to attempt a Hide check. Total cover or total concealment usually obviates the need for a Hide check, since nobody can see you anyway. Your Hide check is opposed by the Spot check of anyone who might see you.
You can move up to half your normal speed and hide at no penalty. When moving at a speed greater than half but less than your normal speed, you take a –5 penalty. You can take a –20 penalty on your Hide check to hide while attacking, running, or charging.
If someone is observing you, even casually, you can’t hide. You can run around a corner or behind cover so that you’re out of sight and then hide, but the others then know at least where you went.
If your observers are momentarily distracted, you can attempt to hide. While the others turn their attention from you, you can attempt a Hide check if you can get to a hiding place of some kind. The hiding place has to be within 1 foot per rank you have in Hide. This check, however, is made at a –10 penalty because you have to move fast.
If you’re successfully hidden with respect to another creature, that creature is flat-footed with respect to you. That creature treats you as if you were invisible (see page 76).
Blend into a Crowd: You can use the Hide skill to blend into a crowd, but doing so conceals you only from someone scanning the area to find you. You remain visible to everyone around you, and if they happen to be hostile, they’re likely to point you out.
Move between Cover: If you’re already hiding thanks to cover or concealment, and you have at least 5 ranks in Hide, you can make a Hide check (with a penalty) to try to move across an area that doesn’t offer cover or concealment without revealing yourself. For every 5 ranks in Hide you possess, you can move up to 5 feet between one hiding place and another. For every 5 feet of open space you must cross between hiding places, you take a –5 penalty on your Hide check. Movement speed penalizes the check as normal.
Sneak up from Hiding: You can sneak up on someone after emerging from a hiding place. For every 5 feet of open space between you and the target, you take a –5 penalty on your Hide check. If your Hide check succeeds, your target doesn’t notice you until you attack or perform some other attention-grabbing action. Such a target is treated as being flat-footed with respect to you.
Sniping: If you’ve already successfully hidden at least 10 feet from your target, you can make one ranged attack, then take a move action to hide again. You take a –20 penalty on your Hide check to conceal yourself after the shot.