To transform into any of the Hulk versions, you'll need to travel to the Nether. There you will have to find Gamma orbs in the zones of Radioactive waste. Bring the orbs to professor Hulk, who is roaming somewhere in the said dimension.
So, the Hulk transformation item is wearable. In that form, a character receives a few positive status effects, like speed, over health, strength, etc.
Clap is an ability that can be performed in Hulk form. Pressing the interaction button while crouching makes Hulk clasp his hands in a mighty blow that creates a sound attack around him.
Hulk smash - use this item to unleash Hulk's rage, smashing his fists into the ground.
Activate all Experimental gameplay features, just in case.