What is the Dimi?
There are two types of harmonicas that are familiar to most people:
One is the 10-hole diatonic harmonica, which is also known as the blues harp. The blues harp is played by a diverse range of musicians such as Little Walter, Bob Dylan, Charlie McCoy, John Popper, and Howard Levy.
The other is the chromatic harmonica, which has been made famous by players such as Larry Adler, Toots Thielemans, and Stevie Wonder.
The harmonica is a rare type of instrument in that you can relatively easily change the pitch layout to suit your imagination, and either of the harmonicas mentioned above can be modified to have an an alternate tone layout. An analogy might be that of a typewriter with an alternate keyboard layout that is an improvement on the traditional qwerty layout.
The "Dimi" is a nickname for both the Diminished Harmonica (an altered 10-hole diatonic harmonica) and the Diminished Chromatic Harmonica (an altered chromatic harmonica).
Either form of the the Dimi is (most typically, but not always) built upon two diminished 7th chords a whole step apart. One blow chord and one draw chord. This results in a blow/draw whole step in each hole, with a half step interval between the draw of one hole and the blow of the next.
The Dimi Chromatic has a slide, which (most typically, but not always) raises each pitch a half step when pressed.