How/when to retract a project from a student? Take, for example, that a student is nearing the end of their thesis project.

The supervisor has an idea for a new method that can help to verify the results of part of the project. He does not think much of the significance of the idea at the time, [writes up a document outlining the procedure] and passes it to the student to implement.

A week later, a problem arises. The supervisor questions whether the idea was in fact more significant than he originally thought - i.e. could form a paper on its own, and has realised that answering the students questions on the specifics of how to implement the said method would take up more time/effort then just doing it themselves.

So, how would the supervisor go about retracting the new project from the student? From the student's point of view, they would have a strong motivation to try and implement the idea that has been brought to their attention since it is directly relevant to their thesis work. Yet the supervisor does not want to be hassled by the student on learning the specifics (that would form the content of the paper), and does not want the student to claim partial ownership of the idea. The student has not yet produced any results using the method that has been passed to them.

The issues that I see are as follows:


If the supervisor tells the student to relax and think about something else instead, they probably would not listen
If the supervisor works independently from the student to produce the paper (figures, text, observing trends), then the student could still claim that they were involved (?) since they would have probably started working on it too
With the way this question is framed, "retracting" the idea sounds both difficult and unethical. Based on the information given here, my advice, and the only ethical path, is for the supervisor to collaborate closely with the student on the work. If the idea is good, the pair should work toward a co-authored publication. This question states that, "since [the student] has more free time on their hands [they] could potentially have some results first." In this case, it sounds like collaboration with the student is very likely to help.

Working with students means teaching and mentorship. It means that supervisors are, "hassled by the student on learning the specifics" and it often means, "answering the students questions on the specifics of how to implement the said method [in ways that] take up more time/effort then just doing it themselves." This is part of the job of an academic supervisor.

Moreover, I completely fail to see why co-authorship and shared ownership of an idea between a supervisor and student is a problem. 

That said, one of the most important jobs a supervisor has is trying to decide when and what to delegate. It was the supervisors job to be more thoughtful and they need to deal with the consequences — whatever they are.