The arms must swing freely on both sides of your swing. Free swinging arms is the byproduct of your position at address. This position should be bending from your hips with a neutral spine as your arms hang freely from your shoulders, a good analogy would be an elephants trunk hanging from his face.
Your arms must be the proper distance from the posture, and you must stand the proper distance from the ball. A tip that I received from Vijay Singh 15 years ago is; when you are in your position, simply take your lower hand off the club and let it hang. If you are the right position the hand will fall in line with the grip, if you are too far away from the ball your hand will fall inside the grip and conversely if you are standing too close to the ball your arm will fall in front of the grip.
So, to allow your arms to swing you must first; be standing the correct distance from the ball, and second; as you make your back swing, if you are a right handed player, you want your right arm to fold 90 degrees on the back swing, and your left arm to fold 90 degrees on the forward swing. This will give your speed and strength during your golf swing.
Remember, there are three power sources in the swing; wrists, arms and body. 30% of your swing is in the arms, to allow your arms to swing freely you must set up the correct distance from the ball and let your right arm bend going back and your left arm bend going forward.
According to VGM