As I remember, most of the early EMD switchers I've run had two switches. The first switch was for "ROAD" or "SWITCH", which determined how fast the electrical system loaded up. In road position, the load regulator starts out in minimum field and slowly balances with the engine speed for a nice slow start. Depending on the loco wiring, the switching position either starts the load regualtor out in maximum field; or creates a basic fixed excitation depending on throttle position. This gives you a faster reponse for kicking cars or switch moves.

The second switch was "AUTO" or "SERIES/FORESTALLING" and determined how the transition worked. In the auto position, you had normal forward transition. In the series or forestalling position, the loco stayed in series and did not make transition. This switch is used to keep the loco in "low gear" if you are approaching transition speed and know you can't maintain enough speed to stay in series-parallel. If your speed drops too low in series parallel, changing the switch to series will also force if to transition backwards to series.

Some, if not most, of the (later?) SW600/900/1200 series changed to a selector lever on the control stand, similiar to the early EMD consoles with a selectro for power(and transition) or dynamics. According to the manual, the four positions were "OFF", "SW", "SERIES" and "AUTO". In the switching position, the electrical system loded faster. Series and auto acted the same as the previous described transition switch, but also were both road excitation which resulted in a "soft start". On switchers with mu, these functions were trainlined so the lead unit could control trailing units. If you mixed road units and switchers in the same consist, if the train slowed the engineer would get a steady wheel slip light on a lead road unit to indicate he needed to momentarily close the throttle to force a trailing switcher back into series.

The SW1500 works a little different because they use the larger D32 main generator, and the traction motors are wired in permanent series-parallel. The selector handle was replaced by a selector switch, which has four positions - "Switching 1", "Switching 2", "Series" and "Automatic". The two switching positions provide for faster loading, and the #2 position raises the normal idle speed. Series and Automatic provide for a soft start for road service, but have no affect on SW1500 transition since the motor connections do not change. This switch is trainlined so it will control trailing switchers that only have forward transition.

SW1000 is a special case. Apparently, the standard design had D25 main generators like the SW900/1200 and have series to series-parallel forward transition. Other SW1000s were apparently built with D32 main generators and are wired similar to the SW1500.

Both the MP15 DC and AC manuals state that these are in permanent series-parallel and have the same selector switch as the SW1000/1500.

SW1500 - Normal idle speed is listed as 344 rpm, switching 2 position is 584 rpm. (run 4?)
MP15DC/AC (op manual) - Normal idle speed is listed as 344 rpm, switching 2 position is 505 rpm. (run 3?)
MP15AC (service manual) - Normal idle speed is listed as 318 rpm, switching 2 position is 497 rpm. (run 3?)

The higher idle speed probably doesn't change the excitation which is still controlled by the throttle position. You still have to get the throttle open to a high enough notch to kick cars, you just don't have to wait for the rpm's to build up.

One of the best engines I ever ran was a SW8 with 65:12 gearing. It didn't go real fast, but it would pull just about anything without slipping or frying the traction motors.