I was not impressed with the one I had until recently. There may be many designs, but the one I had used a tube inside of the overflow, moved up and down with the trip lever. There is an adjustment on the rod that attaches to the trip lever. When it is in the closed position, the tube covers the outlet hole, keeping the water in the tub. By adjusting the tube height, you ensure that it covers the hole when it is closed. If it is adjusted too far one way, it drains slow, too far the other way, and it doesn't cover the drain all the way, and it leaks. Try taking the lever assembly off, carefully pulling up the assembly, and make sure that the adjustment is not loose, and then if it is tight, try lowering it say maybe 1/4" and see what happens. You may have to go the other way.
A pro may have other suggestions, these are from my own observations of the one I had in my old tub.