Orgasm Aftershocks

[Jeanette X]

What causes orgasm aftershocks? Sometimes I find that after the initial orgasm itself, I have these spasms afterward that are quite pleasant, although not as good as the orgasm itself. Furthermore, I only seem to get them with a vibrator, as opposed to simply doing it the old fashioned way. Are these spasms caused by the vibration, or can one achieve them without intense vibration? And are they even "aftershocks" at all, or leser, seperate orgasms in and of themselves?