The low band apply helps keep the sprag in place. The 904 isn't as bad as the 727, but they can still grenade in a similar fashion if they are treated to just the right circumstance. Doing a burnout in first gear and hooking up on the shift to second can cause the over running clutch to spin incredible RPM ripping the sprag out of the back of the case and causing a tremendous transmission explosion.
It's really simple to cure without any extra parts by making SURE it doesn't hook on the 1-2 shift and make it spin on into second gear, but it's safer with a manual valve body, because you can completely eliminate that issue by doing the burnout in second gear. The low band apply helps make everything stronger in that respect. You can tell one has low band apply when you run it up in first gear and back off the gas. If you get gear braking, you have low band apply. If when you back off the gas in first gear and you coast like in neutral, no low band apply. A lot of manual valve bodies don't have it. And IMO, the reverse pattern is mire natural to shift. Pulling back to shift is more comfortable than pushing forward....at least I think it is.
