Masturbacion Anal

I am a 26 year old heterosexual male and considered myself fairly sexual, and sexually experienced. Although I have stimulated a number of girls anuses to mutually delightful effect i have never contemplated doing the same such to myself. And on a recent trip to Spain a spanish colleague tells me about 'Masturbacion Anal' . apparently quite popular in spain i am interested to know about this from anyone who can throw some light on the subject. i haven't yet tried it, though.