@Tamerlane >Plus I like how Sakurakou didn't hypnotize her, implying that she only does it to the people she loves-ie her boytoy.
Assuming they're keeping the hypnosis "realistic" in this manga, you can't just hypnotize someone in a few seconds with a 50-yen coin on a string.
Why Sakurakou can seemingly do this to Takeru is probably because she gave him a post-hypnotic trigger to go back into a trance the moment he sees her dangle the coin on a string, but the first few times she hypnotized him, she would probably have done a long, drawn-out induction which may have involved eye fixation on a 50-yen coin but there would have been much more to it than that, likely using techniques like deep breathing, progressive relaxation (relaxing the whole body by focusing on one area of the body at a time). and deepener (usually involving both visualization of a relaxing scene and a countdown with suggestions of relaxation between the numbers).
If Sakurakou were going to hypnotize Yaotome for the first time, Yaotome would have to be willing to go along with it and it would probably take up a whole chapter if they were going to show the whole technique. (If you're curious about looking at a realistic depiction of a hypnotic induction in manga form, there's a really softcore only borderline hentai manga called Consultation with Sempai that has the best portrayal of the hypnotic process I've ever seen in any form of literature.)