Weads, you should know from your basic POF that the RPM are a function of IAS and weight. Higher speed or weight needs more disc loading. So, in an ungoverned helicopter in auto you demonstrate that the RRPM for a given weight at high speed then, at steady speeds, the slower you go the RRPM decrease. Show this to your student early in the flight then repeat at the end and note the difference with weight reduction. The important point from this is that the minimum weight allowed for flight is so that you will always have the minimum RRPM allowable for autorotation should you have the engine failure( provided you lower the lever promptly!)