Not that SS5150 didn't do a good job, but let me take it a little more detailed....
The 88 series was a gear drive. It had two shift levers, the range lever, and the speed lever. The range lever also contained reverse. The larger tractors had three ranges: High, Medium and Low. Then in each range you had the choice between gear speeds 1-6. The trans still had the the IH Torque Amplifier, but it was changed to an electric shift and incorporated into the speed transmission. So, you could shift between speeds 1-2, 3-4, 5-6 with a simple "slap" of the shift lever from side to side.
shift plate.jpg
When they (CIH) went to a full powershift trans. All they did was add some clutch packs to the STS and boom, full powershift. When SS5150 talked about the "jump" between some of the gears, that is when it shifted between low and medium and medium and high. Single lever in the cab, forward gears numbered 1-18, two ranges of reverse. You could choose rev 1-2, and go to neutral and back to another notch and choose reverse 3-4.
Typical Magnum Interior (9).jpg
The transmission housings on the 88's and magnums were the same. The early Magnums actually used the left over housings. Kinda neat to see one with the IH and original casting/part numbers on it. Several of the parts are interchangeable.