Okay, I can’t speak for this extremely fucked up example, but in general it’s very simple:

1. see shape which sufficiently resembles (partially) naked woman, maybe in suggestive pose
2. neuron activation
3. you are now horny

There is no need to imagine any fucking, seeing the image is enough to get you into the mood, looking at it long enough (or at different images for long enough) will get you most of the way there, and the hand can take care of the rest