Why do Motors Get Hot When Undergeared?

[Revorocks]

Hi,

I've been wanting a definitive answer to this for ages, I gather it's to do with efficiency but I'm finding it hard to proove to someone.

I'm having this argument with some guy on YouTube, yes I know there is no point but it's fun :P

Basically it all started from this video

[YOUTUBE]http://www.youtube.com/watch?v=AkSmbVdyYhM&lc=ObAeBxswFuGpIboWkKDnUjEfRXy Wzt4JUiByIGyyWE4&feature=inbox[/YOUTUBE]

I made a comment saying it was undergeared, and for months now I keep getting people messaging me saying I'm wrong, and undergearing doesn't make it hot etc.

However I've learnt that

Hot motor + cool esc = Undergeared
Cold motor + hot ESC = Overgeared
Hot motor + hot esc = Way overgeared
Warm everything = Just right

Of course it varies a bit with the terrain and your driving style but as a general rule of thumb that's it?
Or am I wrong?

I want to know if and why a motor gets hot when it's undergeared?
I thought it was to do with a motor operates most efficiently when under a certain load, so least energy is lost as heat.
However this guy on YouTube is saying it's most efficient under no load, which I disagree with..

Basically, who's right, and why.

Thanks