Robert:

When you no longer need the weight of the flooded negative tank, it is blown dry with compressed air, and then valves to the outside sea are closed. The negative tank now has no water, but the air in it is at the outside sea pressure level. This higher pressure air is then bled into the boat till they are at equilibrium.

If you use the negative tank till you are at depth, the negative tank will be at very high pressure, and a lot of air will be bled into the boat. This can be uncomfortable, and can risk the bends once the crew surfaces again. Also, if the boat internal air is at too high a pressure, it can make it dangerous to rapidly open the conning tower hatch and get lookouts up upon surfacing - people have been literally blown out of the conning tower from this air blast. This means you have to slowly bleed the air out - making the u-boat very vulnerable while sitting on the surface without lookouts. This is on reason the negative tank was not normally blown dry at depth.

I believe the technique you describe the is a very brief tap of high pressure air done before the dive to make sure tha boat is pressure tight and therefore ready to go under. This is different than bleeding the negative tank once you are under water.

Kurt