I was on Space Cadet Camp at Bruggen in '87 and spent the week 'helping' as a Flight Line Techs on 31Sqn.
On the first day I was told exactly where to stand on the grass bank to ensure that as the Tonka turned after exiting the HAS I was knocked over (no Reheat needed).
A couple of years Later I used to enjoy watching the newbie Refuellers do their first Rotors-Running Refuel on the Wessex, the fore fuelpoint was just behind & below the Starboard Exhaust. They used to come running in all grins and then stick their head right in the efflux......
I think in both cases it is surprise that caused the falling over rather than the force of the efflux, especially in the Wessex case