to say, the energy of atoms in this state increases as they migrate from the trap center, making the trap center a point of minimum energy, the definition of a trap (see blue curve in the figure at left). Conversely, the energy of the |m=0 state is unchanged by the field (no trapping), and the |m=1 state actually decreases in energy as it strays from the trap center, making the center a point of maximum energy (the inverse of a trap, green curve in the figure)(What figure?)(figure in "Talk"). For this reason |m=-1 is referred to as the trapping state, and |m=0,1 the non-trapping states. From the equation for the magnetic field interaction energy, it can also be seen that the energies of the |m=1,-1 states shift in opposite directions, changing the total energy difference between these two states. The |m=-1|m=1 transition frequency therefore experiences a Zeeman shift. With this in mind, the RF evaporative cooling scheme works as follows: the size of the Zeeman shift of the -1+1 transition depends on the strength of the magnetic field, which increases radially outward from the trap center. Those atoms which are coldest move within a small region around the trap center, where they experience only a small Zeeman shift in the -1+1 transition frequency. Warm atoms, however, spend time in regions of the trap much further from the center, where the magnetic field is stronger and the Zeeman shift therefore larger. The shift induced by magnetic fields on the scale used in typical MOTs is on the order of MHz, so that a radiofrequency source can be used to drive the -1+1 transition. The choice of frequency for the RF source corresponds to a point on the trapping potential curve (upper dashed line in the figure at left) at which atoms experience a Zeeman shift equal to the frequency of the RF source, which then drives the atoms to the anti-trapping |m=1 magnetic sublevel and immediately exits the trap. Lowering the RF frequency is therefore equivalent to lowering the dashed line in the figure, effectively reducing the depth of the potential well. For this reason the RF source used to remove these energetic atoms is often referred to as an "RF knife," as it effectively lowers the height of the trapping potential to remove the most energetic atoms from the trap, "cutting" away the high energy tail of the trap's energy