What it is
When carp are cruising, bubbling under the film, or taking mixers that someone else has thrown, a bait on the surface is the honest method. A controller float (or a bubble float, or a simple legered controller) gives you casting weight without sinking the hookbait.
Feed a few mixers or pieces of bread, wait until they're taking with confidence, then put a hookbait among them. In a heatwave, fight them hard enough to land them, keep them in the water, and don't make a five-minute photo of a fish that's already gasping. Some waters ban floating baits — read the card.