Townshend is playing block chords spread between the two keyboards of the 1968 Lowrey Berkshire Deluxe TBO-1 organ. The output of the organ is fed into the audio input of the EMS VCS 3 mk1 synth. The first bit of processing to be applied to the organ sound is a low-frequency oscillator (LFO) controlling the frequency of a voltage-controlled filter (VCF), using a sine or triangle wave shape. In other words, the synth is turning the tone of the organ from mellow to bright, up and down automatically.