Boom support
The topping lift (at the aft end of the boom) supports the boom when the main is down. Slack it when the main is up as the main will support the boom. Vang and mainsheet also will need adjustment.Boats are different...some will have to keep some tension on the topping lift, some will allow the TL to be completely loosened. Experiment. If you have in-mast furling, find the point at which furling is the smoothest & mark the TL at that point.