There are two ways for a flash to output less than full power. Generally speaking, monolights achieve lower output by reducing voltage to reduce the light intensity, meaning no change in duration. Speedlites achieve lower power by reducing the flash duration. That means a Speedlite at lower power is great for stopping action (and why hummingbird shooters use a group of several Speedlites at low power to get enough light with that short duration). A monolight is generally not a good choice for stopping action. There is an affordable exception - the PCB Einstein 640 has an 'action mode' that allows reduced duration. See this:

http://www.robgalbraith.com/bins/mul...=7-10053-10715