The early 78's like th eone shown above had a two giant gears on the end of the haulback. The big case directly to the right of the guyline drums. A set of planetary gears controlled the speed of each gear which was driven by the main driveline on one side and a hydraulic motor on the other. Its kind of hard to explain without many pictures, which I really should scan one of these days. The end result was a yarder that was simple to operate although far too complex and grossly overengineered in some respects. The operator had a trans conrol for forward and reverse and another lever to slack and tighten the haulback and/or the mains. With the rigging running out, push the lever forward and the haulback ran slower than the mains and the rigging slacked on the fly, pull the lever back and the mains slowed, tightening the rigging. You didn't have to ride any brakes or slip any clutches although each drum had an air clutch and a big band brake they were just for stopping. You could park the machine with the rigging in the air and it would stay there with the brakes off and the clutches engaged if you set the lever right. It would also slowly creep in or out if you didn't and I've seen more than one haulback drum snarled up as the line slowly creeped out after the rigging was on the ground. I've never done that of course.
The newer 78A's had a large doubled ended and very expensive hydraulic motor in place of the haulback planetary gear that was quieter, faster and easier to repair but also prone to failure. at least on the machine I ran. When that happened you better have deep pockets.
That's a very brief and rambling explanation and its a lot easier to do it than it is to explain it.
Someone once told me that the Washington yarders were designed by engineers that had left Boeing. I don't know if that is true or not but some of the things I saw on the 78's and 108's made me want to believe it. I think that's one of the reasons that Madill lasted so long. You could fix them with a big hammer, cold chisel and a crescent wrench.