Genital heartbeat

Refers to the pelvic floor contracting when the penis undergoes stimulation, the contracting movement results in the said term.

Janice : I'm not gonna stop edging you
Ian : Janica, wait! Its twitching!
Janice : What's twitching?
Ian : My dick! Argh, it sounds like my heart beating
Janice : Genital heartbeat!