The zero-gravity roll is a roller coaster element where the track twists 360 degrees as it rises and falls. The effect gives the rider a weightless feeling during the inversion.
Theme Parks - Misc, Entertainment - Theme Parks, Circuses and Fairs
In the case of "The Incredible Hulk", the train accelerates from 9 to 40 mph in two seconds through the 150 foot long gamma ray accelerator into the zero-g roll. The start is so intense that some riders like myself actually blackout.
Links: www.universalorlando.com, rcdb.com, virtualglobetrotting.com