Diastolic dysfunction likely contributes to all cases of congestive heart failure and is solely responsible for many. Existing cardiac support devices largely ignore diastolic dysfunction and may exacerbate it. Current diastolic devices in development rely on either extensive extraventricular fixation or intraventricular implantation with complications associated with blood contact. A diastolic recoil device is proposed that pneumatically locks to the outside of the heart wall. The end-diastolic total biventricular pressure-volume relationship (EDTBPVR) was used to evaluate, in vitro, the ability of a recoil device to modulate filling mechanics through pneumatic locking as the method of fixation. The pressure in a model heart was incremented and the corresponding volume changes were measured. The heart model and device were pneumatically locked together using a vacuum sac to model the pericardium. The diastolic recoil component shifted the EDTBPVR towards lower pressures at low volumes, providing up to 0.9 kPa (9 cm H2O) of suction, demonstrating enhanced diastolic recoil at beginning diastole. We conclude that pneumatic locking appears to be a viable method for a recoil device to engage the heart.