There are rubber gaskets on the upper and inner rims on the platter in Air Force series turntable. They form a seal to hold vacuum between the vinyl and platter. When the vacuum is off, the gaskets lift up the record a little bit. There is a small gap between the vinyl and platter mat. When you tap the record surface with the needle on the record, there is a thumping noise.
When the vacuum is on, the suction will press the record onto the platter mat, leaving no gap. The record now fuses with the platter. The platter being fairly heavy, light tap on the record will not cause any audible noise from the speaker.
Thanks for the explanation! Much appreciated.