Have 2 players with different Helicopter Flight Models.

Pilot (Normal Flight Model)
Gunner/Copilot (Advanced Flight Model)
Attacker/Enemy (Person who's going to damage your helicopter)

Reproduction for Exploit #1:

Create a helicopter (that has Control switching) preferred Blackfoot.
Pilot and Gunner/Copilot get in the helicopter.
Pilot (Normal Flight Model) Unlocks controls and throttles up 'LShift'.
Gunner/Copilot (Advanced Flight Model ) Takes controls and presses throttle UP 'LShift'.
Outcome: Helicopter can lift off even before the main rotor spins up to full speed for liftoff for the pilot

Reproduction for Exploit #2:

Works only when the helicopters main rotor is not working causing the helicopter to fall. (Get hit by AA)

1- Pilot (Normal Flight Model) lifts off, gains altitude and unlocks the controls.

2- Enemy/Attacker shoots an AA Titan at you, breaking your Main rotor causing you to fall.

3- Gunner/Copilot (Advanced Flight Model) takes controls of the helicopter while it's falling, making it stop midair for a second and completely stopping the Main Rotor spin.

TIPS for better effect: Pilot throttles down 'Z' while falling.
Gunner/Copilot: Constantly releases and takes controls of the helicopter.

Exploit used in popular MP Missions - King of the hill.

https://www.youtube.com/watch?v=z7dMIOzMSmI&feature=youtu.be