Have checked this one for a while now, waiting for someone better qualified to reply. None forthcoming so here goes. This has been around a LONG time and in the early sixties there was a fairly long-winded series of letters to Auntie Mary's Flying magazine "Air Clues". There were more lift arrows, drag arrows, thrust arrows and general aero-dynamic theories than one could shake the proverbial stick at. Finally it was nailed by - I believe - someone from Boscombe Down who added the magic ingredient INERTIA. He postulated that this was related to the earth's mass and not to the block of air in which the aircraft was operating. The downwind turn therefore required a nose-down pitch input to compensate for the relative speed loss. The normal visual cues (rather than instrument reference) which most light pilots used would not provide the necessary input and the resultant speed decay could, and did, occasionally, harvest its reward !
That's all from memory and it may not be terribly accurate. The originals may still be available but I doubt it
