Ah no wonder couldn't find it was searching on 'Ducati crankcase saver' silly me !!! Description says Case saver fits 900 SS monster 748 916 st2-3-4 4S Would it also fit 749/999 models ?
Prevents case damage from chain snapping as mentioned but also damage caused by a very slack chain. I have seen a 916 case worn away to the point where two grooves were cut into the case and oil had started to seep out, all caused by a aforementioned very slack chain. Up to that point I had never seen that happen.
Don't do what I did - buy a case saver for my 750 and then dilly-dally over fitting it. Fit it straight away. If you procrastinate, you'll be very sorry. Just like I was