Watch the Chord Substitution: 1 online guitar lesson by Fareed Haque from Bebop Improv Survival Guide
Principle 1: Any chord can become a dominant, and still retain its function.
For example: If we have a simple ii-V-I in C major ussually that'd be D min7 to G 7 to C maj7. However if we want to 'jazz up' the sound we might choose to play D7 to G7 to C7. It still functions the same, the ii is still ii the V still V but now we have II7-V7-I7. Way groooovier. Especially if we add in 9ths 13ths etc to sweeten up the voicings.