That's actually an interesting video. Not quite my cup of tea because I'd prefer full nudity and a position that leaves her whole body more exposed, and a dom who is a little better actor himself. But a few things are happening here.
1. I don't think she likes the whipping itself. Like a lot of masochists it's not the pain itself she likes, it's the fact that someone has the power and cruelty and willingness to inflict the pain on her and that she can't get away, perhaps partly because of her enthrallment to the situation and partly because she's fucking tied up and can't escape.
2. There is an interesting transition at about 4 minutes where she starts to flinch less. I think this is where the endorphins are really starting to kick in and her masochistic appreciation of the whole scene is starting to overcome her fear of the pain, and possibly the pain itself. Note her body is continuing to react to the strokes, but she is reacting more subtly.
3. Once the endorphins kick in the whip is still creating strong sensation, but now the pain aversion is muted by the masochistic euphoria of appreciating her situation, and the whip strokes are still vaginal sensation, stimulating her sexually. She is appreciating this aspect of it more now.
4. All through the scene we hear her willingly agreeing to her master's (poorly acted) ritual debasement of her. But toward the end I think it's clear why she is enthusiastically putting up with and even volunteering for this, and it's the euphoric high that overwhelms her when the pain has awakened her masochistic desire. She has to go through the agonizing build-up before that happens though.
I find this interesting because she has basically enslaved herself by embracing her addiction to masochistic euphoria, which requires her to endure that build-up period when it's all just pain and humiliation before the "good stuff" kicks in. And once the good stuff kicks in for her, part of it is knowing that she will crawl back to ask for this again because she can't help herself.
I honestly don't think there is much if any acting in this clip, and it's a good illustration of how masochistic desire works.
