TL;DNR version: Enter the MIDI instrument number (minus 1) in the edit box portion of the drop-down list.
You can do the same trick in MouSing, on the MIDI tab main window.
We actually forgot this trick, but the capability was recently rediscovered by a tester. (The help file will be updated in the next program update to include a MIDI instrument table as a reference.)
MouSing MIDI depends on pitch bending, which varies in quality of implementation by audio device. In particular, MouSing-MIDI works best with large pitch bend ranges, beyond the plus-or-minus two semitones in the General MIDI specification. With large pitch ranges, MouSing doesn’t need to start a different note because that produces a note attack sound– not very theremin-like. If you hear a break in the sound when using MouSing-MIDI, that’s what is happening: you hit a note boundary and have started another note with its associated attack.
Useful theremin-like MIDI instrument voices have indefinitely long sustain and little or no attack. We pre-defined six instruments in the drop-down list: Ocarina, Synth(sawtooth),. Synth(square), Flute, Whistle, and Choir Aahs. We left out a few that might be useful, so refer to the table in this article and experiment.