Custom Transitions?

[TeaBag]

How can I make some?  It seems like maybe the lighter shades fade out first?  Does the amount of gradient affect how long the transition is?

EXAMPLE FROM RTP

[Yami]

You can change the time that transitions last, so it doesn't matter how many shades you have that it will be just as long, but think about it like this

Totally White---------------------------------------Totally Black
|---------------|---------------|---------------|---------------|
Start---------------------------------------------------------End

Start/End might be the other way round I forget, but how it is is like
imagine that the time from start to end is however long the transition lasts.
and however close to black or white is where it shows the next map.

I'm not prolly doing a good job at explaining it.  Sorry X_x