I have a software metronome which randomly skips beats (a design feature).
It is fun to play through a difficult passage, and wonder when the metronome is going to "cut out" on you, and get instant feedback on whether you bridge the gap correctly. Here is an equivalent training technique: set your metronome to silent mode (flash only), and hand it to a helper, who taps a pencil in time to the flash, skipping beats occasionally.
