Jacksterback - A memory that hits you while you're getting off. It could be

A memory that hits you while you're getting off. It could be from years ago or just a few minutes ago.

I was jacking off and suddenly I was back in 5th grade getting called out by my teacher.

I had a flash of the time I got kicked out of the house.

I was getting off and I remembered when I got caught by my brother.