What’s the cause of the characteristic jerk or momentary loss of power felt when a WDM–2 accelerates?

What’s the cause of the characteristic jerk or momentary loss of power felt when a WDM–2 accelerates?

The WDM-2 loco, like most diesel-electric locos, has several configurations of its traction motors that are used as the loco accelerates from rest (Series Parallel - Series Parallel Shunt - Parallel - Parallel Shunt, and weak field configurations). There are three important transitions: At 30.8-39km/h 2S-3P Full Field to 2S-3P Weak Field, at 48-55km/h 2S-3P Weak Field to 6P Full Field, and at 88-90km/h 6P Full Field to Weak Field. For the transition at 39km/h (it can actually happen anywhere between 30km/h and 45km/h), the generator's fuel supply is cut and it is momentarily switched off to avoid sparking and strain on the switchgear. This momentary loss of power can usually be felt throughout the entire train, and is very characteristic of this loco. It is much stronger than the jerks or blips in the acceleration felt at the other transitions of the motors. (This is also seen in other similar Alco models; railfans have reported the same distinct jerk in Alco diesels used in Greece, for instance.) The WDP-2 loco has one transition at 55-62km/h although it is not quite as pronounced.

Source - IFRCA.org