If the starting motion is not directly linear, true. Inertia will make it go in a circle. However, if it's a linear motion, like a vertical bounce of some unspecified manner, there's not much of a circle. However, other bits likely are circling or in a different linear motion as a result of not being on the same line as the linear motion, delay due to their separate inertia, and how they're connected. so you still can't just reverse frames. Very rarely can one just reverse the frames, and usually only with something super simple. ...and even then, you can make it more interesting by not reversing by adding smears/stretched in-betweens for a bit of oomph to the motion.
Nice little tip. (Certainly more concise than my nerding out)
If the starting motion is not directly linear, true. Inertia will make it go in a circle. However,
