I use it when I am playing a 2-5-1 progression, which is basically what 4hisglory explained ealier. For those of you who don't understand chord symbols, here it is spelled out (not necesarrily the exact chord symbols 4hisglory used):
Key C LH/RH
D / F#-A-C-E *half-diminished chord in RH, together with LH makes a D9 chord
G / F-G-B-D *G7 chord
C / E-G-C *C major chord