mesecons/mesecons_movestones/doc/movestone_sticky/description.html

Movestones are effectors that push the blocks in front of them. They move along on the right side of a mesecon wire track. Sticky ones also pull blocks.
A sticky movestone trying to move into, or push other nodes into, an unloaded block doesn't move.
A sticky movestone trying to pull nodes from an unloaded block moves but leaves them behind.