A good way to ingrain this feeling is to bow your knees outward at address. There's not much lower-body action in Mark's swing, but he gets great extension on the backswing and stays behind the ball well at impact. Calcavecchia's isn't the prettiest action, and he sometimes sacrifices balance for swing speed, but he's been one of the game's best drivers for more than a decade. Calc can take that to the bank. His free-swinging style requires an absence of tension.
Tension is a killer.
Try hovering your clubhead at address or, better yet, incorporate a waggle into your preshot routine. Both help the body, arms and hands to stay loose.