Hard rule is don’t forcibly put a rabbit with their belly up (on their back), because this can very likely trigger what is called a “trance”, which is a state where the rabbit stops moving/struggling as a last-ditch effort of enormous stress. They say it comes from a rabbit trying to pretend being dead so a predator loses interest, and some rabbits can die from the remarkable stress trancing puts on them. Back then, people thought it was okay so they’d do it to clip nails and just play with their rabbit, but now we know it’s very wrong and cruel to do it, so avoid doing so.
I think this would also apply to the ball curl, so generally just don’t pick up your rabbit, as the vast majority of rabbits hate to be picked up (reminiscent of being taken off the ground by a bird predator). There are exceptions, but I wouldn’t risk it. It’s much less stressful to just try and get your rabbit acclimated to regular holding by baby steps and taking it to the threshold your rabbit is tolerant with (some rabbits are less tolerant than others).
It can start with just putting your hand under the belly, and then retracting it, sothe rabbit is used to that sensation. Then doing just a one centimeter lift, and set back on the ground. Small steps!
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.