The simplest of the five rules, and the one everybody has heard whether they know the name or not. Its condition is a single test — the chord is a dominant seventh — and its transformation is a single number: move the root six semitones. The quality does not change. What comes out is another dominant seventh, on a root as far from the first as it is possible to be.
Why it can be a rule at all
Two dominant sevenths a tritone apart share exactly two notes, and they are the two that matter: the third of one is the seventh of the other, and the seventh of one is the third of the other. Everything that made the first chord want to go somewhere is still in the second chord, in the same register, doing the same job. The two notes that changed are the root and the fifth, which are the two a bass player and a pianist between them can simply decide about.
The consequence a listener actually hears is in the bass line. A dominant resolving normally drops a fifth; the substitute resolves by a semitone instead, sliding down onto the chord it was heading for anyway. That descending semitone is the sound people mean when they say a passage sounds like jazz rather than like the tune it started as.
What the engine does with it
Because the condition names only a quality, this rule fires on every dominant seventh it can find, which on a blues means eleven of them at once. Nobody plays that, and the page is not suggesting it: what a rule running everywhere shows you is the full inventory of places the move is available, and choosing two of them is the arrangement. Read the before and after rows below as a menu, not a score.
What it leaves alone
Notice what does not change in the tables below. Minor sevenths, major sevenths, sixths and diminished sevenths are all untouched, because the condition is not "a chord" but "a dominant seventh", and the engine will not stretch a rule to cover something it was not declared for. The diminished seventh sitting in bar six of the blues stays exactly where it is.