It's second nature to them as all the speed they have put into the ball now comes out. Now not everyone does what you describe (speith and westwood) come to mind but to me it's a result of how hard they are swinging.

A drill i have been seen is people hitting balls with an impact ball. In order to maintain extension you need to squeeze your elbows together with the ball or it will drop. Check it out!

Ha, yes. The pros are very good and have been practicing for years, but they've been practicing the right things and those are things I'd like to at least kind of aim for.

Gonna look up some video on the Impact Ball!

Clay is great. I like his videos a lot. It's hard to explain, but I have plenty of lag, just not much, if any shaft lean and not much extension in the follow through. My hands are WAY ahead of the clubhead on the down swing, then at impact, the clubhead catches up really quickly, but then it goes past my hands. Generally by the time I get to the "straight line release" point, the club is already a little bit past my arms (as in pointing higher than my arms are, not in a straight line). I'm not casting at all because the club doesn't release ahead of my hands. I do have a pretty fast swing (110 with the driver) and I think a lot of that comes from the lag I have, which I attribute at least partially to the grip pressure. So I guess I'm just not sure how those things go with that extension in the follow through.

They are pros and have had years of practice and coaching

The grip should be loose except the last 3 fingers of the lead hand (left hand for a righty). You have to have enough lag and proper release for the club to catch up to your arms at about 4 feet past the ball or so, at which a point that the club will pass for the follow through.

The arms and club do stay straight for a second.

Clay Ballard does a great job in explaining the release and I think he calls it the "straight line release". You should be able to find him easily on You Tube.

I do pay for his site and think it's worth it. I think the way he explains things are better than most.

A lot of amateurs swing up and down, so as the club passes through the ball they continue to lift the club head up toward the sky to try to get the ball in the air. Good players and pros hit down through the ball and swing the club head low toward the target, allowing the loft of the club to get the ball in the air. This is pretty easily accomplished on the range. Just try to swing the club low and towards the target.