Gripping the pommel (leverage grip). Seems to happen in two ways: directly behind, presumably for point control and to give the sword a good thrusting push, and also on top of the pommel, presumably for maximum leverage in the cut.
Both hands on the grip. (baseball grip) Seems to be the invariable position for Tag and Longpoint, also for Meyer's sole Wrath position. This can vary from the hands being close together at the guard end of the grip to the offhand being directly above (put not gripping) the pommel. Interestingly, the Tag positions mainly seem to have room between the hands while the the Longpoints seem to be mostly right together.
Now, obviously there is a major caveat to this: there seems to be wild variety in hand size and grip length in the woodcuts. However the hand on or off pommel seems pretty distinct irrespective of sizes. Compared to Mair where I see only one clear example of the hand leaving the pommel (see below), mostly when the hands are together in Mair it's only because the grip is so small that the pommel can remain in the hand.
If I had to propose a theory of why this is happen I would only state the observation that having the hands together in the Longpoint position allows you maximum "slinging" rotation of the sword from the upper positions into the cut. You do not bend your arms presenting a slimmer profile to the opponent in the Longpoint. Loosening the offhand off the pommel in the cut also appears to give you a very minor additional reach. Interestingly the under cut positions do not seem to adopt this position, suggesting that leverage of the handle is more important for the under cuts.