Jonny’s ondes Martenot on performances of How to Disappear Completely is improvised, but in a consistent way to fit the structure of the song. Jonny almost always records loops at the same parts of the outro, and they’re usually loops of descending lines. But the notes are certainly improvised each time — in fact, on each performance he tends to add subtle variations throughout the entire song. I’ve never heard it played the same way twice.
During most of the song, Jonny re-triggers his loops to keep them in time with the band (on his favorite looper, the Akai Headrush, he double taps the footswitch to stop and then start the loop again). But during the outro, he allows the loops to fall out of sync for a more unstable atmosphere. Though the rhythm is varied, the pitches are always part of the scale – A, F#, and E are all favorites. So when Jonny semi-improvises over the loop, the harmonies move between consonant and dissonant while staying in the key of the song. Jonny often records one loop before the climactic ascending line, then disables it and records a final loop just after he finishes the ascent.
Jonny sometimes loops just a 3-beat segment, as he does for the penultimate loop during Radiohead’s BBC Radio 1 performance in 2000. For that performance, the loop contains the song’s distinctive descending-third motif, which adds to the feeling of complexity as the 3-beat loop weaves in and out of sync with the 4-beat measure.
The final loop is usually a bit longer (often a measure or two), and the rhythm is usually less precise than on the other loops. In later performances, he often stops recording this loop while still holding a note, creating an abrupt jump each time the loop resets. Jonny never overdubs during the verses of the song (presumably it would get too muddy), but he sometimes overdubs on that final loop to create an especially complex asynchronous texture for the ending of the song. You can hear this during the band’s performance in Prague in 2009: three different layers are still audible after the band has left the stage!
The “acoustic” performance with Thom in Macerata is a good example to study, since Jonny hands and feet are both visible in fan-footage. Jonny also uses his original student-model ondes Martenot at that show, so he isn’t restrained by the physical limitations of the Analogue Systems French Connection, which a little more cumbersome than the real thing. But he does add some extra flourishes in that version to fill out the texture, so it does differ a little from the full-band version. Hope this helps!