If the transfer is interrupted at any time after commitment and before reaching conclusion, the selyn flow is disrupted instead of terminated by tapering off, and the effect is somewhat like a rubber band snapping at full stretch - there is a backlash of energy. Channels are trained to take that backlash into their own nervous systems, protecting the Gen or renSime. The interruption of selyn flow after commitment is technically termed "shen" and comes in a variety of intensities from mild shock to death.
At any moment prior to commitment, a channel can withdraw from the transfer process smoothly and without side effects. After commitment, shen will result.
(Source: Index card file. Sime Surgeon - http://www.simegen.com/sgfandom/rimonslibrary/surgeon/SURGEON1.html) The point in transfer when the channel commits himself to the transfer and allows the selyn flow to begin. This is the moment at which he generally switches his attention from the Gen inward to himself.
At first, channel matches his own field to that of the Gen, preventing selyn flow and observing, then lets his field drop to move into commitment.