Place yourself on your fists and keep your body straight and relaxed as possible. Have a partner lift your legs to different heights and drop you and your job is to relax and to maintain your breathing and your tension at the minimum required to avoid changing your form. Next have your partner lift your upper body up (my students tried to lift me by the ears and it didn’t work well for them) and again dropping you at different hights. Remember that when working hard on hard you need to spread your awareness in your entire body so that the impact will be able to transfer through you without ending inside. Keep breathing and loosen your fists when it is not required to tense them.