The spin of an arbitrarily oriented electron precesses in the presence of a magnetic field (see Feyman lectures 10-7). The energy states also split due to magnetic fields (see Feynman lectures 12-4). I am trying to understand how to think about spin precession, spin flipping, and spin alignment. Let's say we are near absolute zero with 0 magnetic field and a proton is spinning in an arbitrary direction. Then a magnetic field is turned on making a -45 degree angle with the proton's spin. This proton should begin to precess around its axis. The magnetic field however, has created 2 energy levels, one lower energy level where the spin is oriented parallel to the magnetic field, and a higher energy level antiparallel. How do these energy levels relate to the proton spinning and precessing at the -45 degrees? If some energy is withdrawn from the system by lowering the temperature, then the proton should want to flip to the lower energy level. So, does the proton flip to +45 degrees and continue to precess or does it align directly with the magnetic field or something else? If it does flip, or anything other than precess, than what is the quantum mechanical description of the spin of the proton changing its orientation? Thanks for your help.