This is a summary of what I understood about the substitute dominant from a conversation with my colleague Rafael Delgado Buscalioni, who is proficient in music theory. The idea is how to build “another” dominant chord. Say we have a regular, major C chord: C-E-G. The dominant is G major: G-B-D. A typical choice is to include the subdominant in the later, thus adding tension – this leads us to the dominant seventh, G7: G-B-D-F. (this is the way 7th chords entered Western music, it seems!).
By keeping the “tension” notes, we are led to the Secondary dominant, Db7: Db-F-Ab-B.
This is why: