Delia used a keying unit with decay function to trigger the Jason oscillators.
According to the BBC’s own Monograph in-house publication from January 1963, two keying units were constructed at that time. One would drive seven Advance N1 valve signal generators and the other was rigged up to the 12 Jason oscillators Delia mentions.
However, Monograph documents the original unit as having 12 keying circuits that would simply switch on the different signal generators and yet could vary the decay time (fade out) for each of these notes. A later, eight-switch version appeared that could alter the attack (rising) aspect of the signal too, as well as the decay.