I believe what is meant by that is this:
1- While standing, straighten your bowling arm by your side.
2- Now face the palm up so that it is pointing forward.
Notice how the bowling fingers are positioned toward the leg (as opposed with palm down, which places the thumb closer to the leg).
So, with the bowling ball in hand, keeping the hand in the position as outlined in point # 2 above, if the bowler can maintain this position until the ball is almost ready to be released, it will place the hand under and behind the ball for an optimum delivery, enabling leverage, which in turn imparts natural revs to the ball.
To be able to accomplish this can take quite some practice for some people, while it is natural for only a few.
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Regards,
BowlingWolf