Grab releasing is a technique similar to chain grabbing, except the user releases the victim rather than throwing them. Grab releases are the most abusable in Brawl, and not in Melee or Smash Bros. 64, this is because you could not normally release an opponent in 64, and most character's had one attack that was fast enough to counter it in Melee. In Brawl characters ground release if the opponent escape when pummeled for some frames and air release if the opponent escape while not getting pummeled. Some characters can force a ground release others not, it depends on pummel speed.
There are 2 types of grab releases. A ground release where the user experiences 30 frames of lag, and the victim also (with an exception of DK, Ness, and Lucas) experience 30 frames of lag and land close-by on the ground. And an air release, where the user experiences 30 frames of lag and the victim (with an exception of Jigglypuff) suffers 50 frames of lag, but usually move a very far distance from the opponent.
Grab release comboes come into existence when a character has either a very short distance air release, two examples being Wario, and Meta Knight, or experiences additional lag from a ground release, two examples being Ness and Lucas. Others, like Fox and Falco, cannot do anything when released until they reach the ground. This can sometimes also be a disadvantage.
When ground released, both Ness and Lucas suffer from 40 frames of lag rather than the normal 30. This gives the opponent 10 frames or 20 for Bowser to either regrab them if it's Marth, Charizard or Donkey Kong(this last by carry release), or attack if anyone else or those three.
When ground released, DK suffers from only 20 frames of lag, rather than the normal 30, so he has 10 extra frames to counterattack the opponent.
When Bowser grab release (both air and ground release) an opponent, Bowser only suffer from 20 frames of lag, allowing them to chaingrab or attack out of any grab release. When an opponent is released out of it, Bowser only suffers from 20 frames of lag, rather than 30, allowing to dash-grab the opponent when they ground release or attack.
When Jigglypuff is air released it only suffers from 49 frames of lag, allowing it an extra 1 frame to attack out of an air release. This allows it to use rest if the opponent gets close enough but is not essentially very useful, however Jigglypuff still releases a very short horizontal distance despite high vertical grab release.
Wario, who is also known for having grab release issues, experiences the lag of most normal characters and has a normal ground release. However his air release does not send him any horizontal distance at all, leaving him extremely vulnerable for 20 frames in front of the opponent.
Many have argued about whether grab releases should be allowed in competitive play, since they usually much more potent than most chaingrabs, they usually end up as infinites or 0-deaths on the victim.
Well known grab releases are Marth's grab release 0-death on Ness and infinite on Lucas and Yoshi's 0-death on Meta Knight (Yoshi should react quickly with the forward aerial finisher, otherwise it can result in a counter-attack with Meta Knight's powerful up special and counter-KO Yoshi if Yoshi is at high percentages) and infinite on Wario.
It is unknown why Ness, Lucas, Jigglypuff, DK, and Bowser all have different grab release times than the rest of the cast. Most people believe it was to "nerf" or "buff" them intentionally, but this can never be proven.