Can someone explain "split shifting" to me with a SM420 4 spd and a 2 spd axle?

I assumed it was 1st gear, axle low then 1st, axle hi
2nd gear low the 2nd hi...

But I got to thinking this can't be correct. The axle split is much larger than this would allow.

I've always had to get under the truck and shift it at the valve, never had the luxury to try it out. Always drove her around in low range on the farm.