One slang name for them is honking stoppers. You hold onto them when you stand up and honk on the pedals to accelerate, particularly on the back handlebars of a tandem, which had no brake levers. If you ever rode a two-male tandem for speed with the pedals in sync, you would appreciate them. In addition they afforded more hand positions for the stoker on a regular tandem too, with pedals out of sync. You can use them on one side of the handlebars of a track bike that has only one brake.