In this particular version of a press, because she is lying on an unstable surface, she has to contract her leg muscles, as well as her gluts and abs in order to stabilize her trunk while performing the movement with her arms. Whenever doing any kind of chest pressing exercise, whether it be with dumbbells or a straight bar, you should always start the lift from the chest and bring the weight up over the head in a curvilinear manner. When working with dumbbells, one way to maximize recruitment of the chest musculature is to internally rotate the arms while bringing them into the mid line of the body and over the head as demonstrated here. This movement pattern recruits more muscle fibers, because you are performing all of the movements that the chest musculature is designed to perform.