If the chain is slipping, it means it's temporarily riding along a larger diameter arc around either the cog (more likely) or the chainring. It's like switching to a bigger cog on a derailleur bike -- something has to compensate for the increase in arc length.

How long is the chain? I'd guess there should be a rule for chain length similar to the rule for a derailleur bike, except that big-big is just the gears that you've got.