It's a favourite trick at Old Sarum. At a safe height, reduce power to below that needed to maintain level flight. Attempt to maintain level flight. As the speed decays feed in more power to achieve a target speed while still maintaining level flight.
In something like a C152 try doing it with a target speed of 60 kt, then 55, 50, 45 kt.
I doubt this is accurate but I seem to remember getting it down to about 42kt with pretty near full power applied.
You need to do some clearing turns to make sure there's nothing underneath you.
The exercise is very similar to doing a power-on stall but with more accurate height and speed control.
Mike