The Pilot Core Tool allows you to drive posts into difficult ground conditions.
It removes a core of earth out of the ground prior to driving in your stakes, using the Easy Petrol Post Driver. When the ground is hard, bone dry or rocky it enables the job to take place in any condition, and it speeds up the overall time of driving each post in.
Also recommended when the ground is made up of heavy clay, or chalk.
This article also gives some handy tips on how to drive posts in when you have hard ground.