A quantum instruction is still issue by a classical computer and the results of computation are stored in a classical computer. The instructions that are issued are quantum instructions that can solve a very specific part of a problem that can be solved on a quantum computer far more efficiently than on a classical computer. Not all problems fall into this category.
Classical computers turn on and off circuits to do different kinds of computation. The quantum computer does the same thing, but turning on the right circuits cause quantum entanglement.