This one kills me every time.
It's just a quick pan shot with this lady in the foreground straining at her excercise, but not moving. Man is the joke ridiculous.
The Kegel muscles are the muscles of the pelvic floor. They can be strengthened to aid in childbirth for women, help control urinary incontinence, and help prevent premature ejaculation in men. Generally the conception is that for women, exercising the Kegels can help them get "tighter".
That sort of makes this a dirty joke, but just really funny.
(BTW the dirtiest joke to make it onto a prime time cartoon remains the "Sneed's Feed and Seed" joke from The Simpsons)