somn has it right. It was an improvment over the single stage clutch and not as good as a independent pto. I have had both, the independent pto rules but you can make a two stage clutch work just fine.
Dave
The XX00 series Kubotas are very capable, the M6800 one of the best. I have a M4900 MFD and regularly handle 1200 to 1400 Lb. haylage bales with no problem. Kubies are on the light side so you do need to weight them but they are rugged and in general no problems.