Dental plaque forms on all surfaces of your teeth, especially in-between teeth and along the gumline. It is important to focus on these areas during brushing by directing your toothbrush bristles toward your gums just like you would with your manual toothbrush. You should be able to feel the bristles along your gumline.
When using a rotary toothbrush, you can follow the same general pattern of brushing that you would with a manual brush, the only difference being how each area gets cleaned. The toothbrush is electric, so it does all the work; all you have to do is manipulate it in the correct orientation around each tooth.
Rotary toothbrushes have brush heads that are sized to clean each tooth individually, so you should slowly rock the toothbrush back and forth along one tooth at a time. The bristles should also overlap onto the gum tissue, because it is important to clean the area where the tooth and gum meet. Be sure to pay attention to the bristles and feel them gently surrounding that tooth as you clean it. Be careful not to push too hard with your toothbrush. Not only will it put force on your toothbrush and potentially break or damage its motor, but it can also cause abrasion to the gum tissue.