Minecraft wiki entry for Trap_design, associated text: Suffocation. A player or mob whose upper body is in a solid block will take suffocation damage. This can be accomplished by dropping sand or gravel in the target. However, players are pushed into an open space if possible to avoid suffocation damage. If a player can quickly mine out the block causing the damage, that player will stop taking damage and may be able to escape. Using pistons to push the block into the player is an effective method if you know exactly where the player will be.