Refer to figure.
“G” is an abbreviation for acceleration due to the earth’s gravity.

The best way for a pilot to recognize low G is a weightless feeling similar to the start of a downward elevator ride.

Helicopters rely on positive G to provide much or all of their response to pilot control inputs.

The pilot uses the cyclic to tilt the rotor disk, and at one G, the rotor produces thrust equal to the aircraft's weight.

In a low-G condition, the thrust and control authority are greatly reduced.

Although their control ability is reduced, multi-bladed (three or more blades) helicopters can generate some moment about the fuselage independent of thrust due to the rotor hub design with the blade attachment offset from the center of rotation.

However, helicopters with two-bladed teetering rotors rely entirely on the tilt of the thrust vector for control. Therefore, low-G conditions can be catastrophic for two-bladed helicopters.

Specifically, an abrupt forward cyclic input or pushover in a two-bladed helicopter can be dangerous and must be avoided, particularly at higher speeds.

During a pushover from moderate or high airspeed, the helicopter experiences a low-G condition as it noses over. Thrust decreases, leading to a loss of control over the fuselage attitude. The pilot may not immediately realize this. Roll may be induced by tail rotor thrust or other aerodynamic factors. While the pilot retains control of the rotor disk, attempts to correct the roll by applying cyclic inputs may lead to excessive fuselage and rotor disk angles, potentially resulting in mast bumping.

In addition, turbulence, especially severe downdrafts, can also cause a low-G condition and, when combined with high airspeed, may lead to mast bumping.

Ways to avoid Low-G mast bumping:

• Avoid abrupt forward cyclic inputs in two-bladed helicopters. Accomplish a rapid descent by lowering the collective.
• Recognize the weightless feeling associated with the onset of low G and quickly take corrective action before the situation becomes critical.
• Recognize that uncommanded right or left roll
• Recover from a low-G situation by first gently applying aft cyclic to restore normal G before attempting to correct any roll.
• If turbulence is expected or encountered, reduce power and use a slower-than-normal cruise speed
• Use a flight simulator to learn to recognize and experience low G conditions that result in mast bumping