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.
Foot radical on the left - pictograph of a foot with the knee or thigh atop a footprint. Indexes 踹 in the foot family with 踢 kick, 跳 jump, 跑 run. Marks the character as a forceful action of the foot: stomping or breaking open a door with a heel.
Right side 耑 supplies the sound with a drift: duān to chuài. 耑 is the older form of 端 (end, beginning, tip), and rare on its own today - pure phonetic role here. The "tip" sense of 端 faintly chimes with how a 踹 lands on the heel-point.
