Usage of the Jake brake is a CDL school topic (or should be). It was when I went to CDL school.
The Jake brake only applies braking effect to the drive axle, from engine compression. The trailor receives no braking effect to its axles, thus the trailor will try to push the tractor, on dry pavement, this should be no problem, as long and both drives and tandems are getting good traction.
Problems can occur on wet (rain/snow etc) or icy pavement. If the Jake is used on this type of surface, the drives may lose traction, putting the tractor into a skid. The trailer will be pushing the tractor, and if there is any angle between the tractor and trailer or if the drives loose traction unevenly the trailer will try and swing out to the side.
The more weight that is on the drives the better the drives will maintain traction, but the heavy trailer would mean a greater push by the trailer.
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when going down a steep grade you're geared down. Is that not basically producing the same effect as the jake and possibly causing a jack-knife?
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Basically yes, assuming you a "geared down" without the jakes, the engine compression is still applying a certain amount of braking effect to the drives.