If such a force persists or is strong enough, the spinning prop will NOT be deflected in the direction the force is applied, but in a direction 90 degrees further around its rotation. 

Example: 

If you try to push the bottom of the prop disk forward, and the prop is spinning anti-clock as viewed from behind, the force will be effected 90 degrees anti-clockwise from the bottom, i.e. on the left of the prop disk. So, trying to tilt the prop-disk over backwards, will result in the prop disk deflecting to the Left! 

In paramotors, this happens mostly when the pitch of the motor changes. If your prop rotates counter-clockwise (as viewed from behind) as most pusher props do, then if you (and your motor) pitch upwards (i.e. lean backwards), then the motor will yaw sharply to the left, such that you may end up facing your left wingtip!!! This is dangerous, as your thrust is no longer forward (direction of wing's flight), but toward that wingtip. The right wingtip will dip down sharply (being pulled by the thrust), and the wing will roll over to the right (starboard) and start turning to the right, but you are facing (and being pushed) to the left! This leads to a disastrous situation. 

I have seen this happen to many pilots without them being aware of its cause, nor its remedies. Most survive purely due to instinctively releasing the throttle. 

The change in pitch (which leads to the precession or left yaw) could be from any of: 

•sharply applying brakes - the wing slows down, the pilot/motor swings forward of the wing and, having high attachments, leans over backwards, i.e. pitching upwards. •going suddenly from cruise (or glide) to full power - same effect as above. •from a siophic sitting upright (or leaning forward) position, to suddenly leaning back and lifting the knees to a more comfy position. 

Unfortunately, sometimes pilots do all three in one motion, which can lead to a catastrophe!!!!! 

Another instructor had a student spin 16 times under his wing in under 2 seconds. The wing carried on flying out to sea, and the lines had 16 twists; the pilot had no input to the wing. He killed the engine and started unwinding, the wing carved a gradual curve back to shore, but he struck a building before regaining control of the wing, while flying downwind. A long hospital stay and some serious injuries later, plus a motor almost written off, due to Gyroscopic Precession! 

Here is something every paramotor pilot MUST do: 

•Take a bicycle wheel and hold it by the axle. •Ask someone to get the wheel spinning quite fast. •Then try to tilt the wheel over in one direction... 

You will be surprised by the forces and the inevitable result... The only thing a pilot can do when precession kicks in, is to gradually back off the power and wait for recovery and level flight, then slowly re-apply power. Just hope this does not happen near the ground. Unfortunately, it is most likely to happen just after launch when you least can afford to back off the power. 

POP QUIZ: just to see if you understand the cause and effect of GP: Imagine doing a low flypast and the left side of your propeller-cage brushes against a bush on the ground (heaven forbid this really happening), and so tends to swing the motor and pilot sharply to the left. What will the resulting effect be from Gyroscopic Precession?