Difference between constant gain and constant power? (and a suggestion)

for those of us mere mortals, whats the difference between constant gain and constant power?

id like to suggest that a more intuitive term for this would be "crossfade"

and as a feature suggestion, i would be highly intuitive to be able to apply this by simply selecting the two audio clips and hitting "X" on the keyboard to apply it

i have seen it in some Audio DAWs and it works well.

thanks