Freestyle stroke linear or curved path?

In looking at the animation on www.swimsmooth.com I see that the hand follows a straight path front to back. This is probably better seen when looking up at the swimmer from under water. If you put a ruler to the path it is pretty much a straight line. Now, I've seen and I've read posts that talk about a non-linear path. I guess the idea is to always try to accelerate new water rather than applying force to water that you've already accelerated. So...why do they show a straight path. And, if curved is better, what is the best path?