This article presents the development of a hybrid model with pneumatic lag effects for the smart seat cushion which can be extended to any active aircell cushion. Smart seat cushion is one such rehabilitation device which reduces the risk of pressure injury formation by automatically performing repositioning exercises for wheelchair users. Operation of the smart seat cushion device can be greatly enhanced by accurate prediction of pneumatic line lag during its various modes of operation. The proposed model combines a linear model from literature to capture the effects of pneumatic line lag and a bang-bang scheduling controller to enable efficient use of pneumatic components in the smart seat cushion device. The resulting simulation is validated by conducting multiple runs of uniform inflation experiment with the device, and comparing the closed loop behavior of the model. The hybrid modeling allows accurate prediction of pneumatic line lag in the smart seat cushion during various modes of redistribution and offloading. The modeling accuracy and the prediction of switching times for discrete states can be improved by data sampled at a higher rate from the device.