It’s useful to disconnect the chain so you can check the resistance in the bottom bracket without interference from the back wheel. It’s easy to do this on derailer bike.
Dismount the Chain
If the bike has more than one chainring shift the chain onto the smallest. Push the bottom jockey wheel forward to relax the chain. Lift the chain off the small chainring and move the chain carefully inside the chainrings so they can spin without interference.
Is the bottom bracket loose Enough?
Spin the cranks. They must turn freely with no clicks or catches and minimum friction in the bottom bracket.
Is the bottom bracket tight Enough?
Grab a crank and pull it from side to side. If there is discernible movement – play – the bearing in the bottom bracket is too loose.