Before the event, there were concerns about Forster's body actually coming back to life, because this was the 19th century and people didn't know things, but as the Newgate Calendar (a record of London executions) informed its readers, his sentence was that he should "hang until he be dead," which effectively meant that should he return to life, he would be marched directly back to the gallows.
The terrified imp on the far left is the only one in the picture with an appropriate reaction.
The experiment was carried out by Giovanni Aldini, who, by the time he zapped Forster in public, had not only mastered his core technique, but developed a theatrical flair. First, he applied the electrified conducting rods to Forster's face, which caused Forster's jaw to quiver and grimace horribly. His left eye popped open. Aldini then applied the rods to Forster's torso, causing the body to jiggle and contort and even blow out a candle. What, you couldn't flex the jaw and make it talk in a funny voice? The audience had to have been disappointed by that, but way to show some restraint there.
Wait, we take that back. Because nothing could have prepared audiences for the grand finale, in which Aldini shoved a conducting rod into the cadaver's rectum, causing it to punch a fist in the air (really, this is the type of climax that no one is prepared for).
"You keep your eyes open, people. This is the world."