As much as anything, forced feminization has a large degree of control. There is a pure form of dominance that extends beyond the BDSM means to something transformative. At the core, the Mistress is removing many of the aspects that are so much a part of the ego and identity, things that so many take pride in, and are replaced in form, with softer things she desires.
Humiliation creeps in by some degree, and while by far not the most perfect picture, her serious demeanor and his sense of shock do convey a degree of power - "See? See what I am changing you into? Do what I say or I will expose you to others..."
So as she holds him up to the mirror, so he must look back at the image being reflected, it is, in essence, her wielding her power and control, letting him know that he is but a mere puppet on her strings.
This is where the belly full of butterflies begins. For the majority of cases, those who desire feminization tend to find a willing partner, albeit, often by the surprise of discovery, and so often the partner is the last to know. Whether it is search history, hidden magazine, a stash of clothes, the "secret" is discovered. After the shock period, there sometimes is acceptance or sometimes revulsion. It can be a deal-breaker as such a thing forever changes the notion of one's identity in the eye of the other.
But at the same time, and where I bring up the butterflies, there can be a nervous aspect as well. For many like myself, this is a somewhat part-time and infrequent activity. It would perhaps be more so with a willing partner. At the core of my desires is not so much the dressing, but being dressed. But again. that harkens back to the adage, "Be careful what you wish for..." Power can be an aphrodisiac and addictive. Perhaps she starts to want more and deeper still.
Feel free to drop a line to me and we can see where it goes