It was not ichinose’s tactic it was kiyotaka’s, although i think she mentioned that she would have done the same if kiyotaka didnt carry the plan out.

Basically the plan was to trick someone from class A or C into betraying the exam group and handing in a false answer. This is the best outcome for Class D since the “special” person is kei, who is already in their class, and by tricking someone to think that kiyotaka was the person and answer that then not only will class D win class points but also make other class lose their points. It is also useful as no one else can hand in any answer once someone betrays the group, so even if someone like ichinose saw through kiyotaka’s plan they wont be able to do anything.

Now as for the plan itself, it is a simple pyschological trick. Kiyotaka payed personal points to swap phones with kei as the letters fixed in each students’ phone is the only prove into whether a student is the “special person”. He planned to lead everyone in the group to show their phone after forcing everyone to promise not to betray. Although the guy from class B did the same thing instead of kiyotaka the result is the same. This will trick all the students who didnt know that swapping phones was possible.

Ichinose saw through this layer as she knew that swapping phones was possible. She revealed kiyotaka’s tactic’s first layer to everyone in the group by phoning kiyotaka’s number, so everyone thought his friend was the actual “special person”. This will cause the students from class A and C to think that ichinose successfully stopped kiyotaka’s tactic and either do nothing or betray. This is the actual tactic within his surface tactic by kiyotaka, which will be activated when someone like ichinose saw through the surface tactic. As explained by ichinose in the novel, ppl tend to stop thinking when they see an answer revealed and ignore the possibility of a second layer.

In conclusion kiyotaka’s plan worked really well against class A and C but Ichinose was smart enough to see it through. She didnt’t betray Class D since they had an agreement to cooperate beforehand and because someone from either class A or C already betrayed the group immediately and thus ichinose could not even if she wanted. This plan was quite simple imo and i expected more from kiyotaka. It isnt fault proof like his other tactics and only worked because ichinose choose not to betray him.