(ET) what does the foot pedal sw-4 do?

[Charlie <medievalist gmail com>]

I had a switch go bad in my foot pedal and have been studying the
closest wiring diagram I've been able to find (thanks, Geo!) to my
late-model I-5.

I understand how the double throw start switch works, I understand how
the 1A and 2A switches control the 1A and 2A contactors to drop the
toaster out of the motor armature circuit, I understand how the FWS1-4
switches drop the field resistors into the motor field circuit -
that's all pretty straightforward.

I found my bad switch (it wasn't sw-4) and that's not a problem any more.

But what does the sw-4 switch do in the late model large frames?  I
haven't been able to figure it out.

In puzzlement,
--Charlie