Crab...I remember at Valley it was possible on certain wind direction and w/v (around 20-25kt) days to take a Wessex out to Snowdowia and find a near vertical cliff face with updraughting air,
Sit in the hover along side the cliff face at just over half power, in the hover, zero airspeed showing.....then over a period of about a minute or less, slowly have to pull to max power to maintain position, before having to dive away lowering the lever and gaining fwd airspeed to recover. It was repeatable over and over again.