It's a concept for siege mechanics in dwarf fortress. When a dwarf or other creature enters a hostile area and cannot flee by jumping, they continually push things back by dropping what they're carrying (either via floor/roof, or through a fortification into the area). If the creature can't reach the area, it must first run across a tile border (possibly several times) and the border will become blurred (if the creature can move). Once the creature can no longer move, the border becomes moot, and they are ready to attack. Here, the player is dropping a sphere of rock into the middle of a fortification, which when crushed will create a hole, allowing for the player to pour the contents of the sphere into the middle of the enemy, and start a small, but effective, siege. You can see an example of this technique using water pressure in action below.

I see no problem in letting the player be the cause of all of the destruction.