In general, IMO, our body just cannot create very fast straight linear motions. Thatís why, Federer and Nadal rotates straight arm before impact. I didnít see any pro that can use straight linear motion before and during impact. Maybe you can provide some videos?
I don't think they can. All they seem to do is to manage to get the racket face perpendicular to the ball trajectory at impact and control the curvature to some extent, so as to meet the trajectory smoothly (as I had said before). You seem to think of linear as parallel to the sidelines. I think of it more as a measure of local curvature of the arc at impact (what you would know as reciprocal of the second derivative).