踹 is a hard, forceful kick — think kicking down a door or stomping. For a lighter kick (like in soccer), use 踢 (tī). 踹 implies aggression or force, 踢 is neutral.
memory
The left side is 足 (foot) — it's done with the foot. The right side 端 (duān) gives the sound hint. A forceful kick that 'ends' something.
