Mecanum Drive - Help

[MrForbes]

Let's see if I understand it.....going forward or backward (in the direction of the wheels), the rollers do not need to turn relative to the wheels. Going sideways (perpendicular to the direction of the wheels), the rollers always need to turn relative to the wheels.

The "lost motion" of the rollers turning when going sideways, indicates to me that this is a less efficient way to move, compared to going forward/backwards with the rollers just "going along for the ride".

Do I have it right? My old brain has trouble grasping new concepts, so I would not be surprised if I don't understand it all....but I do have a little mecanum wheel here to play with.