Prometheus thinks nothing of catching Thor in his fist, holding him just tight enough to keep him from escaping his grip. Definitely not tight enough to hurt him, which is why the sudden burst of electricity catches him by surprise.

Despite being the Titan of Forethought, he did not see that coming.

And wow, does it hurt! It's not the smiting strike of one of Zeus's forged lightning bolts, but it's powerful, divine energy that burns where it strikes him. He yelps and opens his hand instinctively, letting Thor fall out, while the rest of him staggers backward, the electric current traveling downward to the ground below his feet, where it finally dissipates.

"I thought we agreed no powers!" Prometheus sounds more confused than anything, cradling his hand to his chest. "What happened?"