You are telling the amp to make more power then it can possibly hope to achieve. It cannot physically do it without causing major distortion to the signal. Thats why you have noobs buying 400 watt amps powering 600 watt subs thinking they will be safe, but when they have it all set up and its not hitting as hard as they want even though they are pretty much maxed out on power output from the amp, They start cranking stuff up even more. They think its louder but its actually distortion and it leads to blown subs.
Theres impedance rise and voltage drops to account for so even a reputable 1000 watt amp would probably only make 600-750 watts after rise and voltage drops. 400 watts would be a lot less which is pretty much like tickling the sub. Thats why most people encourage headroom with power. More loudness without the distortion.