He was relieving himself on a work detail off the jail campus. The guard wanted to make sure he knew where Luke was and that he was still going, so he was supposed to shake the bush. Luke had tied a string to a bush, so that he could buy some time for an escape. Having a hard time explaining this, as it is very early out here. I can't have coffee for another 20 minutes.
Luke should have just served his two years for his petty crime and moved on to make his life clean and better. But that was not in the script.