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If they're holding on to you and not the bike then it will be like this. Some bikes worse than others for slippy seat and no grab handles. It can help if they can get one hand on the back of the fuel tank to brace themselves against that rather than all their weight on you. You can also get some stuff called wonder web for the pillion seat that makes it really grippy so less sliding.
Took Mrs Hammer as a pillion for the first time today, which went ok (no-one fell off) but I hadn’t anticipated how much a physical workout it was going to be. Breaking/downhill especially!