The third sensor in a triple sensor action functions between the normal bottom sensor (which sends Note On) and the top sensor (which sends Note Off). Within the key's travel, the presence of the middle sensor by itself sends no MIDI data. But by lifting the key high enough to pass the middle sensor (but not so high as to lift it past the top sensor), you can now press the key down again to trigger another Note On. This means (a) you do not need to lift a key quite as high to be able to be able to repeat it, and (b) you do not need to lift the key so high as to silence it before striking it again. (This second attribute only applies when you are not using a sustain pedal, since with the pedal down, you can always restrike a note without silencing it first.) If a keyboard sends Note Off velocity, it is done by the top sensor, which is the only sensor that ever sends a Note Off, and it is calculated based on the time the key return travels from the bottom sensor to the top sensor. (It's not impossible that it could also do or alter that calculation based on when the key passes from the middle sensor to the top sensor, though I think that is unlikely, and it is certainly not required for the board to generate a release velocity.)

If you used a triple sensor board like a CP4 to play a sound that was in the MODX, you would still get both of the benefits of the triple sensor. You would still be able to restrike the note without lifting it as high, and without silencing it first (the CP4 would still be able to send two MIDI Note On events without sending a MIDI Note Off event in between). If you used the 2-sensor MODX to play the piano sound in the CP4, while the notes themselves would sound the same, you would no longer have the triple sensor benefits which result from the inherent characteristics of that mechanism, i.e. you would no longer be able to restrike a key from lower in its travel, nor (without the sustain pedal) be able to restrike a note (generate another Note On) without silencing it first. In short, the third sensor physically affects the capabilities of the keys, but does not generate any MIDI data of its own.

Thanks for your responses!