Complex sentence: The descending piston is used to compress incoming combustion air for the adjacent cylinders which also serves to cushion the piston as it approaches bottom dead centre (BDC) to remove some load from the bearings.
Split into simpler sentences: The descending piston is used to compress incoming combustion air for the adjacent cylinders. This also serves to cushion the piston as it approaches bottom dead centre (BDC), thereby removing some load from the bearings.