Watch the The b5 Substitution online guitar lesson by Brad Carlton from Guitar Lab: Weekend Jazzer
This lesson briefly discusses the principle of the flat five substitution.
By utilizing the flat five substitution you can create harmonies that are functioning as altered V chords. An altered dominant chord is one in which the 5 and/or the 9 has been sharped and/or flatted.
These harmonies contain a tension which is then resolved you play the next chord.
This is one of the most important principles in jazz: tension to resolution. There are many altered dominant scales and a considerable amount of time must be spent on these sonorities in order to effectively play through jazz changes.
I suggest you begin by learning some altered dominant arpeggios and just stay close to the chord tones.
As you learn to hear these altered tones you will be more comfortable with melodic moves involving tension to resolution.