I recently picked up an indi Zero 6S discharg Tray.

When should I use this? After I've run the batterys? Or just before I charge my cells?

I believe these trays give best results when you leave your cells on them for anything up to 5 days at a time. Best pick up a few more trays

Do not do this to IB cells !!!.They will be destroyed by zeroing to this extent.

How low can you go with IB cells?

I have never seen the idea of bringing Nimh right down, their maintenance is easy, leave as is after a race, before running discharge to 5.4, put on a tray for a few minutes to equalise and charge. Its not rocket Science