Same deal as using Seafoam. I've used it a couple times on cars I fix at work. They were running rich and the valves were all carboned up so I sucked the seafoam right into the brake booster line and it cleaned the motor right up. I've also used water as a cheaper alternative, worked just the same and it was free. Once I tore down the motor there was no carbon on the valves or the tops of the pistons, it works like a champ! Definitely a good way to clean up the carbon.
97 Dodge neon, SOHC 5 speed