The Energy Saver 2 is the size and shape of an AA battery. It substitutes for one of the alkaline batteries in the flash battery comparent. Always start with a fresh set of AA batteries. The result will be much longer lasting AA's. The Energy Saver is for : Canon 430EZ, 540EZ, 550EX; Nikon SB24, SB25
I bet a lot of people don't know what this is for, and neither did I for a long time, so I thought I'd write a review explaining it's purpose.
When you use a speedlight on your camera with an small external battery pack, the flash still requires batteries inside the flash to fire. You can't just take the batteries out of the flash and rely only on the external pack. What the flash does is partially drain it's own batteries first and then suck juice from the external pack as a secondary source. This doesn't always give good recycle times if your AA's are getting weaker.
What this does is trick the flash into thinking there is a battery inside the flash, so it will fire, but it will suck power straight from the external pack, giving you much faster recycle times. You would only use one dummy battery and three real alkaline/NiMH batteries. The in-flash batteries will still be drained of power unfortunately, but that power just isn't being used as the primary source for recharging the flash.