Velocity is speed in a specified direction; Acceleration is a change in velocity. Simple?
MY OPINION - Turning downwind at low level can be a risk when the pilot's perception of a dramatic increase in GROUNDSPEED is compensated by reducing AIRSPEED, which leads to descent, which may be further complicated by raising the nose to arrest the descent; which may result in a major reduction of airspeed if aviation rule No. 1 is breached.
(n.b. Aviation Rule No. 1 - whilst in flight thou shalt not make inadvertant contact with the ground)
