I've watched and participated quite a bit at this point. The only real "what happens after" is that we have twice as much sex as usual for week or two. I do think it requires having total confidence in each other and that there are no ulterior motives at work, that the relationship is the priority and this is just for fun. People struggle with that both with general baggage about sex and fears about their relationships.
As for what it was like, it was just extremely intense the first time. It was just soft swap, but when you've cum in a woman's mouth a couple thousand times, you know her technique inside and out and watching it is a far more vivid kind of voyeurism than you're going to find in any pornography or stage show, up to the moment when she starts working hard to finish him off all the way through to those few protracted gulps and wiping her mouth. I had been too stunned to even pay any attention to my own erection, but got some help with that halfway through. I was rock hard but totally distracted, until she joined in, visibly a mess from sucking off the other husband, and was mingling tongues with the other woman sucking me. That was when I had to stop and walk away for a minute to get a beer because I wasn't going to last another minute if I didn't.
Point is, it was just fun. Since then, we've had a number of visits with those same two couples, and done plenty of full swap with them. Never any drama, and any doubts we've had before we jumped in always turned out to be over-analyzing. We always treat whatever happens as a warm-up act for whatever happens next and just have fun. Sometimes that means one of us is sitting out while the others screw and it's fine. Finding the right mix of attitudes helps.