The used Canon ST-E2 Speedlite Transmitter is a wireless flash controller that can fire up to three secondary groups with variable output ratio up to 8:1. Key features include:

E-TTL II Compliant. The Canon ST-E2 Speedlite Transmitter is fully E-TTL II compliant and can control 580EX (II), 550EX, 430EX (II), and 420EX. While it provides wireless E-TTL II functions, it can also be used to simply trigger secondary flashes with the flashes set to manual power. The ST-E2 also emits an AF-assist beam. It’s significantly smaller and lighter than a flash, so it’s a good choice if you just need a focus assist.

Infrared Triggering. It uses IR pulses to trigger secondary flashes, so unlike radio-controlled triggers, all flashes must have line-of-sight. Direct line-of-sight isn’t strictly necessary indoors—the pulses bounce around pretty well, moving around corners and through light modifiers like softboxes—but it’s a really good idea outdoors. Range decreases with the level of ambient IR, so trying to trigger a flash that’s sitting in direct sunlight is tricky at best.