I know a bit about this fetish. Not that I've done it. Setting up a happy 3way takes work and can be a rare thing. But I have dated men who had this fantasy, and I've looked at the porn (which I suggest you do).
As I see it, the long term partner gets off on the "humiliation" of seeing a younger studly guy fuck you. (There is also a large niche of cuckolding where the newer lover is black, where the original couple are both white. This seems racist to me, but I guess it adds to the pleasurable humiliation for some white men.)
He might just watch, or he might want to guide the other guy's cock into you, while you both verbally abuse him, mock his penis size, etc. He might fantasize about eating the other guy's semen out of your pussy (good in porn, but definitely NOT safe sex).
Personally I think it's kinda gay~ not that there's anything wrong with that! The cuck gets to see another guy's cock at work. He might get more involved and help you to give oral to the guy, etc., etc. But he might just want to see your face suffused with pleasure, watch you writhe around, from a voyeuristic distance.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 61) loving miss pixi (poly, F, 39) since January 2009, living together since 2013
also loving Punk (42, M) since Oct 2015 (he has recently downgraded us to friends)
"Master," (mono, 34), miss pixi's Dom for 2 years