Positive displacement compressors operate under the principle that:

Positive displacement compressors function based on the principle where they utilize a changing volume to move and compress air. The correct understanding here is that they have areas that both enlarge to allow air intake and then reduce to compress this air. This process essentially traps the air in one section and then reduces that volume, which leads to an increase in pressure according to Boyle's Law. Therefore, the concept of increasing area for intake followed by a decreasing area for compression accurately describes how a positive displacement compressor operates.

The other choices do not accurately represent the operational principle of positive displacement compressors: while air does flow from low pressure to high pressure, this is more of a general statement about fluid dynamics rather than a specific operational principle. The notion of velocity changes within confined spaces relates more to dynamic compressors rather than the fixed volume concept of positive displacement. Lastly, while pistons do create pressure through the movement of air, the emphasis on suction alone does not encompass the entirety of the positive displacement mechanism, which relies heavily on the change of volume to achieve compression.