Let me see if I understood this correctly. The bottom position is the only stable position for the pendulum. You give it energy (pushing it, kinetic energy) and now it swings into an unstable position (gains potential energy, loses kinetic) and so it must come back down to a stable position...
When a nucleus is placed inside an external magnetic field, it aligns itself parallel to the field, as that is the most stable position for it to be in. In more technical terms: it acquires an alpha spin.
When you shine radio waves at this nucleus, it absorbs energy and flips over anti-parallel...