The most colloquial description I can think of is that sudden dynamics in the music seem to leap out of the speakers at you. For example rimshots or a hard snare drum hit, or the pluck of an acoustic guitar. It's hard to nod off when listening to such an amp.
In contrast a "slow" amplifier might have an equally detailed sound, but simply puts you to sleep. This might be a good thing if you like to relax to music.
It's all a matter of taste (though my personal taste is that "fast" amps sound closer to live music). If an amp gets too "fast" it can be very tiring to listen to for extended periods, though it could sell very well because of the wow factor during short demos.
In contrast a "slow" amplifier might have an equally detailed sound, but simply puts you to sleep. This might be a good thing if you like to relax to music.
It's all a matter of taste (though my personal taste is that "fast" amps sound closer to live music). If an amp gets too "fast" it can be very tiring to listen to for extended periods, though it could sell very well because of the wow factor during short demos.