The following is from a post several years ago that I described the process to grease the intermediate sheave assy and what happens when it is not greased regularily, likely what happened to your belt. 


The Intermediate drive needs to be greased properly, I don't think the manual is very clear on the procedure but what you need to do is run the cylinder speed up to the max speed, this will open up the intermediate sheave assy, then remove the vent plug from the intermediate assy, it is the plug (7/16 wrench) that is about 180 degrees from the grease zerk then pump grease into the assy until it comes out of the vent plug hole, Make sure you have it at max speed or all of the extra grease will come out of the vent plug the first time it is sped up, and believe me it makes a terrible mess. What happens with the posi-torque drive is that it will open up some under power and start slipping the belt and if it is dry it will not close back up to maintain it's grip on the belt then it will suddenly let loose and slam closed and grip the belt so tight that it will tear a piece out of the belt, So you've got two possible failures here, a burned belt and/or a belt with chunks torn out of it. Depending on how many hours of use it sees a year this needs to be checked every couple hundred hours along with daily greasing.