Need help with this phrase
龜背功為硬功外壯，屬陽剛之勁。此功專練背部，使之堅實，與腹部之布袋功、鐵牛功等法，效用相同，用以禦敵，非用以制敵也。

practising this art, would strengthen one’s back, make it harder, like a carapace / tortoise shell 

扣畢復摩

i would shortcut it to “massage [one’s back] after knocking”
in details, “扣” is

以拇指，扣住中、食二指之第一節，使其第二節骨突出，即用突出之處，向腰部軟當上環扣之

use one thumb hold the first joint of index finger &amp; middle finger, let the second knuckles protruding, use these knuckles to “knock” one’s back
畢: finished (完畢)
復: again, and then
摩: massage (按摩)

以兩手緊按後腰，先向內揉摩三十六次，復向外揉摩三十六次，如此為一轉

seriously, do you believe it?
龜背功:此功專練背部，使之堅實(圖文)
be cautious