The best way to explain a butt cape is it's like an overskirt, but it's generally not fancy (so like no lace or ruffles) and is over pants/shorts instead of a skirt. Or in really extreme cases, underwear and Zettai Ryouiki Grade A/S
That's an overskirt
That's a butt cape
So same concept, just underneath is what's different
About 99.99999% of people will still call them overskirts regardless. For me, at least, there's a distinction between the two