A second gradient simplification (the subject of the question above) refers to the fact that in many references you will see a smaller downward projecting lobe in the slice-select gradient immediately after the RF-pulse. This is called the slice-rephasing lobe. This helps correct for the phase dispersion of transverse magnetization that occurs concomitant with application of the main slice-select gradient. Without this rephasing lobe, intravoxel phase dispersion would occur with resulting signal loss.
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In spin-echo sequences the slice-rephasing lobe is sometimes time-shifted to occur immediately before the 180º-pulse, being combined with the left element of a crusher pair surrounding the 180º pulse.
It should be noted that spatially selective refocusing pulses do not require rephasing lobes because the transverse phase accumulated during the first half of the pulse is cancelled by the second half.
Hutchinson JMS, Sutherland RJ, Mallard JR. NMR imaging: Image recovery under magnetic fields with large non-uniformities. J Phys E 1978;11:217-221
How does frequency-encoding let you stimulate a given slice?