The purpose of the rule theoretically, is to put 9 hitters in the lineup. Arguably to allow pitchers to be single-minded. They get "paid to pitch" - don't get hung up on the word "paid". Since the rule is not limited to pitchers it provides several additional strategies for the lineup.
As for mechanics the best resource I have seen in at
http://www.cactusumpires.com/pdf/lin...t-ASA-NFHS.ppt