They're gasoline-powered inline skates! Via McClatchy's China Rises newsblog, which says:
The skates have a 25cc engine and a small fuel tank behind the right heel, and are controlled by a handheld throttle. But there is no brake! The only way of stopping is a waist-level shut-off button that kills the motor. Skaters likely then go sprawling.What's Cantonese for "ACME"?
Imagine what happens in a crash: The plastic fuel tank catches fire and the skater quickly gets crispy.