Gwen giggled as she felt her cousin struggle. He was fighting tooth and nail to get up, but her butt was just too heavy. Gwen could feel that he was attempting to get his arm free, obviously to activate the Omnitrix. As his pinned arms attempted to shift, Ben repearedly pinched Gwen's ass fat, causing her to giggle.
It also felt kinda good.
Finally Ben spoke.

"This is so gross." Ben said. That made Gwen laugh "Then you should feel right at home.".
"Gwen, what the heck is going on?" Seriously, one minute he had his back turned, then the next, he was being used as a chair by his big butt cousin. Sure Ben knew Gwen could do magic, but this was crazy.
Gwen meanwhile didn't dignify his answer with a response. She had grown bored with his obnoxious voice, so instead, grinning even more mischieviously, she wiggled her booty again. Any remark Ben was giving quickly became muffled, as his face was forced into her crack. Now wedged between her cheeks, Ben began to yell something else. What Ben was saying was anybody's guess.
Gwen meanwhile, would have been laughing(pardon if this seems redundent) he ass off, but instead, something else happened.

Gwen stopped, and a low moan escaped her lips. Each time Ben struggled, he would cause her rear cheeks to jiggle, and as Gwen felt it, she would moan again.
"This feels...good." Ben would have gladly accepted that he had his head in a butt crack, if he knew that his cousin, was actually becoming 'excited' because of it.

Gwen moaned again, this time louder. Her eyes were starting to get glossy. She could also feel a moistness coming from between her legs. Wanting to feel this sensation increase, Gwen wiggled her hips again, shifted her rear again, so Ben' head would go deeper.
Gwen gasped, and as her cousin's legs struggled even harder, she forced her hand in between her vaginal lips. She wasn't entirely sure what she was doing, all she knew was that this felt good. Something about sitting on top of Ben, and with an unbelieveably huge ass had turned her on so much. She plunged her fingers deeper, switching between clenching her teeth, and letting out another gasp. She could feel a pressure building up deep inside her, and she tensed up.
Finally when she couldn't last any longer, Gwen pulled fingers out, and screamed as her first orgasm racked her body.

Gwen would have collapsed if she hadn't already been sitting down. After catching her breath, she looked down at her hand, still covered in her natural juices.

The add was right, this was a pleasure spell.

Gwen couldn't wait to mess with the spell again...maybe, she'll make her breasts big enough to cover The Rust Bucket.