A few posts ago we looked at a simple ii-V7 substitution that implies a secondary dominant over the ii-7 by playing a minor seventh chord up a half-step from the ii-7. This implies a subV7 of V7 because you are playing the related ii-7 of the subV7 (related ii-7s and V7s are interchangable as far as substitution goes).
original
D-7 /G7 /Cmaj7
played
Eb-7 /G7 /Cmaj7
implied
Ab7(#11)/G7 /Cmaj7
Now let's take this same concept one step further and substitute the related ii-7 of the subV7 of Imaj7 for the regular V7. This sounds convoluted but is so simple to calculate on the fly. All you have to do is play a minor seventh chord a half-step above the dominant.
So over this
D-7 /G7 /Cmaj7
play
Eb-7 /Ab-7 /Cmaj7
implying
Ab7(#11) /Db7(#11)/Cmaj7
subV7 of V/subV7 of I/Cmaj7
Very, very cool. Very, very easy to do.