Trampoline is a Scheduler that executes code on the current thread. The way it works is that it adds a code block that will be executed to a queue of the current thread.
If it is used on .observeOn() and .subscribeOn(), it doesn't change the default behavior of the Observable and, thus, it is basically a No-Operation (NOOP).
It is mostly reserved for advanced use when creating custom Observables. An example where Trampoline could be used is to help avoid StackOverflow exceptions when calling recursive code.
Finally, if there is really a need for it, and a developer knows it's something that he really needs, it can be created with a call to Schedulers.trampoline().