keep in my mind this is long fist, not white crane or taiji...you should tense up when you finish your punch so that the muscles protect the joints; I used to do what you describe...and I got elbow pain till I understand that it wasn't the same power as white crane!
when you pratice long fist punch, you should try to think that you want your fist to go through the enemy's body. so there's no pulling back whatsoever.
also, the power comes from the whole body, push with your feet, jerk your waist and gather all that power in your fist to literarily breakthrough the enemy.
(if you don't see what i mean and if no one gives a better explanation...let me know...hopefully, Walter may come by and will tell us everything we have to know
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