The swivel is after "both arms straight." Once you get to both arms straight, you have to finish the swing right? You do this by "swiveling" your entire left arm with a flat left wrist THEN you can allow it to break. Please know that the stronger the grip the faster the left wrist will break after both arms straight. Here is a picture:
Also, Lynn blake has video of him teaching Collin Neemin on his site and demonstrates the swivel as well. My "tap the tire" comment refers to Ben Doyle. If you watched any of the videos on Lynn's site of him teaching a student, he has him try to strike an golf cart tire. If it "bounces" on the tire you are "adding" and leaking to some extent. It should be a nice solid "thud" with the tire moving along the ground.
Allow the left wrist to break in the follow through and you'll find yourself "tapping the tire."
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But the question isn't whether or not you have to do it all the time or whether or not you want to employ it, but the question becomes can you do it? Sometimes, you want to "maximize compression" as Brian says. You need a little more mash and this is how you do it. You can allow it to break a little early but you aren't going to maximize that "smash factor." Brian demonstraits this is a beta video that he put up for free that i'll link to below. Watch the video and he'll demonstrate two similar shots against a fence and you make the decision for yourself which one "mashes" the ball better?
http://homepage.mac.com/brianmanzella/.Movies/segment1bw.wmv
Right about 5 minutes in....however watch the whole thing. It's free and it's good stuff.