Oracle Text

Whenever Aetherplasm blocks a creature, you may return Aetherplasm to its owner's hand. If you do, you may put a creature card from your hand onto the battlefield blocking that creature.

Card Rulings

2/1/2006 Aetherplasm will return to your hand before any combat damage is assigned.
2/1/2006 If you want, you can return Aetherplasm to your hand and not put anything onto the battlefield. The attacking creature will still be considered blocked, so it won’t be able to deal combat damage to you unless it has trample or a similar ability.
2/1/2006 The creature you put onto the battlefield may be the Aetherplasm you just returned to your hand. This allows it to dodge abilities like Deathgazer’s.
2/1/2006 The creature you put onto the battlefield from your hand is blocking the attacking creature, even if the block couldn’t legally be declared (for example, if that creature enters the battlefield tapped, or it can’t block, or the attacking creature has protection from it).