When I disengage the blades on ride-on lawnmower smoke began appear

So I looked where it was coming from - seemed to come from where blades are although I could be wrong . Blades no longer working when I pull up engage button as I would hear them . Is it a belt that's gone? Thing is,when I disengaged blades smoke stopped! I was cutting a garden with grass nearly 10 inches long . It was fairly dry though so my Snapper shouldn't have too much of a problem with that grass although she was labouring for a large part of the cut . Didn't complete job . Then when I turned blades off and smoke stopped the engine was revving up and down so I don't know what that is . Can still drive the machine but blades will not work when engaged and smoke may begin to appear again. I'm a bit unsure of myself in all this .