When electric motors are disconnected from a circuit, I understand that they can spin relatively freely with little to no resistance, only the shaft friction causing resistance. Connecting the motor to a circuit, and suddenly the motor prevents free spinning. I understand the resistance is coming from the motor acting as a generator, creating a voltage and producing current.

My application ideas for this would be to allow for e-bikes to be used as a normal bike; a CNC table that you could manually move the arms when required; an electric skateboard that could be used as heavy normal skateboard.

What is required within a circuit design to allow the motor to remain "connected", however, be able to relatively free spin? My thought process would think that a relay would be required to disconnect the circuit.

Is there any differences relating to this problem whether the motor is DC or AC?

When electric motors are disconnected from a circuit, they can spin relatively freely with little to no resistance, only the shaft friction causing resistance. Connect the motor to a circuit, and suddenly the motor prevents free spinning. 

I understand the resistance is coming from the motor acting as a generator, creating a voltage and producing current.

My application ideas for this would be to allow for e-bikes to be used as a normal bike; a CNC table that you could manually move the arms when required; an electric skateboard that could be used as heavy normal skateboard.

What is required within a circuit design to allow the motor to remain "connected", however, be able to spin relatively freely? My thought process would be that a relay would be required to disconnect the circuit.

Are there any differences relating to this problem whether the motor is DC or AC?