What happens in the booty at the point of orgasm?

Define booty!: Assuming you mean your vagina and pelvis, there is swelling and lubrication followed by intense waves of muscle contractions and then relaxation. If you are interrupted at any point, you may not finish and you will probably have to start all over again. Right after orgams, hypersensitivity is common where continued touching is unpleasant. This resolves with time.

Pelvic Floor: Some muscles of the pelvic floor will involuntarily contract during an orgasm. This includes the muscle of the anal sphincer. If it is the gluteus min/max you refer to, the contractions of these muscles would be voluntary.