You need to lie on your left side first for 5-10 minutes. Then roll over and lie on your right side for 5-10 minutes. When you do an enema the liquid goes directly into the "descending colon" which is on your left side. You give it time to fill with the liquid. Then you roll over on your right side to send the liquid into your "ascending colon" which is on your right side.
Look at the picture on the linked webpage below and it will make much more sense: