I was just about to comment on this haha. I feel like some of you have thought something was about to happen, when it really wasn't.
With a BS, the moves can sometimes mimic sexual positions. I get a feel for the client first, then decide how intricate the slide will be. If he seems laid back, understands guidelines, and is enjoying himself, I've definitely slid in some positions that could be questionable. A thigh slide for example, mimicking doggy or revers cow girl. Also an ass slide, like a hot dog in a bun. The keys to these moves are muscle tension, and only doing it with trustable clients since someone could easily jam it in.
If a client seems off or seems eager to rub himself in areas that wouldn't be safe, he'd get a more basic slide and wonder why his sessions aren't as good as the ones he reads about