Complex sentence: Bottle dynamos typically create more drag than hub dynamos though, when properly adjusted it may be so low as to be trivial, and there is no resistance when the bottle dynamo is disengaged.
Split into simpler sentences: Bottle dynamos typically create more drag than hub dynamos. However, when they are properly adjusted, the drag may be so low as to be trivial, and there is no resistance when the bottle dynamo is disengaged.