I have not read Piggots book but the term Sub- Gravity refers to either =
reduced "G" or Negative "G" loading.

Some people are badly affected by such sensations and may act =
irrationally if subjected to them. A common cause of approach/landing =
accidents is due to such sensations being encountered by pilots in =
gusty/rough conditions and believing that they are stalled. When they =
take stall recovery action close to the ground when, in fact, the =
aircraft is not stalled, you can guess at the outcome