We used to do it that way a bit, but my wife's back has been bothering her for the last year... age catching up. When we did use that position, we only did it for a couple minutes because I went too deep, and it hurt her. The feeling for both of us was great, however. She'll still rarely grab her knees and pull them way back like that, and it's like a special treat.
I still get the best feeling just from standard missionary with her feet flat on the bed. She can move better that way... and boy can she move!
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