My interpretation of 'reverse' sanding is to have the abrasive pushing the fibres the opposite way to 'normal', thereby not allowing them to bend out of the way. This is usually done by reversing the wood rotation on the lathe, guaranteeing reverse sanding, but if the section of the abrasive that is cutting is moving against that direction, you have, in effect, got reverse sanding. The only criteria for that to be the case is that the abrasive is moving faster downwards than the wood is moving downwards. I am assuming you are sanding in the usual 8 o'clock position. When using my bowl sander I can find positions where it spins hardly at all or very fast, depending on which part of the disc is driving the rotation, so it's not necessarily the direction of rotation of the the sanding disc, more the speed and position where it's cutting.
Hope that makes sense and would be interested to hear other's opinions,