What Do The Arms Do In The Snatch?

During the first and second pulls, the arms essentially just connect the bar to the extending body. The primary goal in these phases is to keep the bar as close as possible to the body without dragging to preserve balance and allow optimal mechanics for acceleration and elevation.
The arms should be pulled straight by the weight of the bar, not actively extended. Use the back and shoulders to push the bar back toward the body and keep it close—it will take more effort between the knees and mid-thigh as the shoulders will be farther in front of the bar.
As you’re finishing the extension in the second pull, keep the arms relaxed and guide the bar up close to the body after it contacts the hips by continuing the backward push, then shrug up and back as you finish the extension.
As you transition into the third pull, the arms themselves need to become as active and aggressive as the lower body was in the previous phases of the lift.
Pull against the bar by lifting the elbows up and out, continuing to actively push the bar back toward the body.
Squeeze the shoulder blades and arms back as you turn the arms over to bring the bar into the overhead position, and finish the motion by pushing straight up into the bar over the back of the neck to lock out the lift forcefully.