Piston

Pistons are blocks capable of pushing adjacent blocks, depending on the direction they face. The original version of this feature was a mod posted on the Minecraft Forums by Hippoplatimus. Pistons are not going to be added in Beta 1.5, but in a later version. Pistons will be slightly altered from the original version, and the usage section below only explains the mod's usage.

Usage
The original piston mod is capable of moving adjacent blocks (except obsidian and bedrock), depending on the direction the piston is oriented. To change the orientation, a user would only have to right click on it, and the piston would face them.

When Gravel and sand are placed on an piston facing upward, they will launch into the air, then fall back down into place. Other things that can be launched are entities and items. If the player is in front of a piston facing sideways, the player will be shoved about 3 blocks away from the piston. If the player stands on the blocks that the piston is pushing, the player will fall through the blocks.

Pistons are controlled using redstone currents.