If you can get hold of a copy of Bob Robert's book on legering, there's an explanation of the bubble feeder method. It's intended for still or very slow moving water. The idea is that you use a Kinder egg or similar, which will actually float (just) with nothing in it. i find a small Thamesley feeder with the lead strip works.
Drill a few holes if using a Kinder and attach to the line free running, as you would a bomb (a tightish powergum loop running the length of the egg/feeder and very small swivel, which keeps the top on, is best). Use a tail of 18 inches - 2 ft, a single maggot on a small hookand a soft actioned quiver rod with a fine tip.
Fill with maggots, cast in, just tighten the lineand the weight of the maggots will cause the feederto sink very slowly. Bites come as fish take maggots escaping from the feeder as it sinks, one of which will be your slowly sinking hook bait. When the feeder empties, it will actually start to rise in the water, at which point, it's time to refill and recast. Not a mthodintended for fishing on the bottom, the method works best for fish feeding in mid water, usually in warmer conditions.
On its day, a great method.