The best way to understand what the Amorphous Core is, and also why it's better, is to know what amorphous metal is.
The ribbon tweeter has a transformer with a steel core. Steel is like saying plastic, there are many kinds of steel. A normal transformer has ferrous steel, the amorphous core transformer has a core made from amorphous steel.
Amorphous steel is the same alloy, but cooled at 1 million degrees per second (seriously) so that it hardens with no crystaline structure. By not having a crystaline structure, it becomes much more efficient with much less energy wasted as heat.
The end result of how it sounds in a RAAL makes perfect sense when thought of in those terms. The AM Core RAAL sounds more efficient, like more of the amplifier current is reaching the ribbon element. It's faster and more dynamic, with more low level detail retreival and an overall more engaging and energetic sound.
Personally I think the AM core is a HUGE upgrade from the standard core. When performance gets to the level of the standard core RAAL, any improvement becomes hard to come by ... the AM core gives you that same RAAL sound, just more of it.