Put long socks on. Then pull the front part of the sock forwards. The object is to flick the sock as fast as possible in a controlled way. Once this is done with ease, remove the sock and your body will understand the action that is required. Flat of foot for tournament fighting, ball of foot to indent POWERFULLY. To PRACTICE? Place the ball of the foot on a wall and push off , replace, replace then place . Google "The True Value of Martial Arts for Self Development" or Martial arts as a personal training practice " for more. Good luck sifu.. this is my 54th year. I write to assist you. 🙏✨✨✨🦘