Fundamental to cellular processes are directional movements driven by molecular motors. A common theme for these and other molecular machines driven by ATP is that controlled release of hydrolysis products is essential to use the chemical energy efficiently. Mechanochemical transduction by myosin motors on actin is coupled to unknown structural changes that result in the sequential release of inorganic phosphate (Pi) and MgADP. We will describe how key regions of the motor play a role at the actin interface to trigger different transitions during the powerstroke. We will also present a myosin structure that explains how actin initiates force generation and movement. This structure possesses an actin interface that differs from previously seen pre- and post-stroke states of the motor. The structure also describe the tunnel (back door) that creates an escape route for Pi with a minimal rotation of the myosin lever arm that drives movements. Functional studies allow us to propose that this state represents the beginning of the powerstroke on actin, and that Pi translocation from the nucleotide pocket triggered by actin binding initiates force generation by myosin. This elucidates a strategy that may be common to many molecular machines.