It uses the E510 chip, a specialized MIDI keyboard decoder integrated circuit. The kit is capable of switching between MIDI channels 1 and 2, and (if proper key contacts are supplied) will provide velocity sensitivity.
This little board, unfortunately ,is not all that is required: you have to build up external decoding logic for the individual keys -- one 74HC138 chip per eight notes. Instructions are included with the kit, but you have to make your own layout.
In addition, I built an optocoupler isolating assembly for absolute safety and isolation from the organ's DC supply. I took this precaution because of some high voltages I found from the organ negative to a piece of structural steel near the organ -- mysterious, but no time to fully trace it. Knowing that such situations might not be uncommon in other installations, I thought I'd experiment with the safest possible interface.
This interface used the useless "5-1/3' Quint" stop knob to interrupt the negative bus to the optocoupler assembly to serve as a "MIDI on Pedal" stop. This conveniently put the typical 32-foot "flue" sounds usually used on the synth right into the organ's combination action. Another useless stop, the "All Swells to Swell", serves to change the interface's output between MIDI channel 1 and 2, giving me a handy opportunity to switch (usually) from the synthesized 32-foot "Unterfarz" (flue sound) to a "Bombardier" (quasi-reed). (These whimsical names I have assigned these patches are not at all out of line with the imaginative nomenclature typical of synth sounds!) Of course, these can be assigned at the synth to any electronic sound for those times you are pushing the boundaries of strict organ tone.
This setup kept me pretty happy for a time. After leaving Annunciation, the organ sounds I was coming up with on my synths started to feel quite good for home practice, so the interface (which came with me to Milwaukee) is now fitted to an old but solid 1920's Reuter pedalboard. And I've made it velocity sensitive - I hope that with a bit of practice, this could be useful not just for expression, but to switch between synth sounds -- for example, "stamp harder to add a tympani!" (As time goes on, and organ practice takes precedence over sonic fiddling, I have to admit I haven't exploited the potential of this feature, though occasionally when I do new-age noodles the velocity sensitivity is welcome.)
Just to show that I'm not alone as a crazy electronic home builder, here's a link to Peter Rodwell's MIDI pedalboard page on the International Organ Foundation web site. I first encountered his work when he installed the same Maplin kit I used - now he's found some good inexpensive circuitry from Sound Research, which isn't velocity-sensitive but does have better features for channel selection, etc., depending on the model.