First, let me be clear that I am talking about happily married couples who realize that the wife should be sexually active with other Cock's with the husband finding such fulfillment in it that he cums better from watching her versus just screwing her himself. Lots of couples have issues that they think a kink moment will fix-WRONG. So, that is my parameters for discussion.
Second, it seems that there is some evolutionary part of it with watching the wife, i.e., "egg" be turned into a cum-bucket, if you will, so that all that sperm saturates her. The excitement of seeing your wife turned into a major Slut used solely for sexually pleasure ignites a primal xxxxx within or something with the husband. Let me add that "Slut" here refers to a woman with an advanced sexuality who knows it and seeks to explore it accordingly.
The wife seems to derive real pleasure not only from being a genuine Slut but in knowing how to please her man. When she is being fucked by an other Cock but looks straight into the eyes of her man who is either watching or stroking to it they both find an intimacy that = when he fucks her. The rhythm she ends up in with the penetration of the Meat-Cock while taking in what her show does for her husband sends the Orgasm Wave from deep in her lower-back to a crescendo over her entire body and mind completely. She finishes thinking she has been fucked by two Cock's simultaneously and no DP involved in it.
The variations of being in the room and watching her strip show for the Cock's used to prime them, how the husband may suck the Cock's to "get them ready" while his wife watches and lubes to his prep show, to how she "goes out and fucks" at "random" all are designed to enhance a unique sexual lifestyle practiced by the couple.
So, how do members do it or think about it?