(ET) how does the late model I5 speed control work?

[Charlie <medievalist gmail com>]

On my 1973 I-5, I have a "gas pedal" that contains some switches that
drop resistances out of the circuit of the traction motor armature,
and more switches that add resistances in series with the traction
motor field.

Normally, each switch is opened (or closed) in sequence as the driver
increases pressure on the pedal, and as each armature (toaster)
resistor is dropped out, or field resistor added in, the tractor
surges forward a little faster.  I can feel each distinct switch
taking effect.

However, if I push the pedal all the way down from a dead stop, it
does not immediately jump to the fastest speed - instead it works its
way up through the speeds as if I'd pressed the pedal more slowly.  I
can still feel a "surge" as the 2A contactor closes - even though all
the switches have already been flipped.

Sometimes, when I'm going uphill in too high of a gear, the tractor
will want to stay at the lower speed (unless I press the CC button -
also known as the Power Pulse button - which causes it speed up
immediately).

What's going on?  How does the tractor "know" when it's ready to drop
out the next resistor, if it's not just doing what the switch position
says to do?  I have a schematic (thanks Larry!) and it seems like the
2AH relay and the diode board are somehow involved in keeping the 2A
contactor's coil from being energized, leaving half the toaster still
in series with the armature.  But that's as far as I've gotten...

--Charlie