Query on Rotavating

I moved into a new house a few months ago and am trying to tackle the back of the house, ie it is not a garden

Basically it is earth with a lot of stones just below the surface, ranging from ones the size of your palm to ones that are close to the size of a soccer ball.

I have hired a power digger for the weekend and was wondering if anyone had used one before, here is the digger I hired

http://www.hss.com/g/62516/Power-Digger-Petrol.html

I have never used one before so wondering what we happen when it comes across a large stone, ie will it just make a noise and I stop the digger and shovel out the stone........then should I be using this, ie will I destroy the power digger?

I know that this would be better but they have not in stock for hire this weekend http://www.hss.com/g/62526/Cultivator-8hp-Petrol.html

The website has a pdf which is the operating & safety guide.

This says that running into debris could cause serious damage to the digger.

Debris is listed as bricks, large stones, wood and wire so that answers my question.