Sub-gravity sensation

I need to give a season opening briefing to all the pilots in our club next
week and naturally want to highlight safety issues. During preparation for
this meeting I've come across an accident report that attributed the cause
of the fatal accident to "Sub-gravity Sensation". There appears to be a book
available on the subject by Derek Piggott, but I cannot get ahold of this
book by Wednesday.

Anyone every heard of sub-gravity sensation? What is it?

Thanks,
Rod