Excitation modifies energy levels and spin phases. At the quantum level, a single proton jumps to a higher energy state (from parallel to anti-parallel). The consequence on the macroscopic net magnetization vector is a spiral movement down to the XY plane.
In a rotating frame of reference, the net magnetization vector tips down during excitation. The flip angle is in function of the strength and duration of the electromagnetic RF pulse.
The net magnetization vector can be broken down into a longitudinal component (along the Z axis, aligned with B0), and a transverse component, lying on the XY plane.
During excitation, longitudinal magnetization decreases and a transverse magnetization appears (except for a 180° flip angle).
Longitudinal magnetization is due to a difference in the number of spins in parallel and anti-parallel state. Transverse magnetization is due to spins getting into phase coherence.
If we consider an excitation with a 90° flip angle, when the RF transmitter is turned off: