Basicly, the best usage of the 'T crossing' would be during a line battle, by combining the T with the Weather Gauge advantage. It allows for THIS manuever, which was most-feared:
A quick turn downwind can allow for the tactical manuever of slipping into the gap between ships. (Often several hundred feet, to allow for the ability to manuever somewhat) Hence why having the weather gauge was vital. It allowed for control of the engagement.
This manuever would allow the crossing ships to fire broadsides out of both sides of their ships, as well as all of the decks, at an enemy who can only fight back with bow and stern chasers.
(And I say all decks because due to riding low in the water due to damage, or choppy seas.. a ship catching the wind from the side will rock from side to side, and often the lowest deck of guns would be submerged during the rolls and had to remain closed and not used during combat.)
Sorry for all of the pictures, I felt it was the best way of explaining things.