Consult Google.com and Youtube.com.... Ur best friends in this regard.
As for my case, Its a good thing you asked. Like imran said, Experience. Its not just the p1222, but nearly every competition level subwoofer/amplifer are under-rated.
Heres an example. The competition your entering requires you to use an amp that puts out around 400rms@4ohms, meaning the sub ur using either needs 400rms to run at its optimum level or is under-rated at 400rms (but then it wont sound like it would with higher power fed to it)
but now, u go in with a p1222 that can easily take on 800rms (check google. people have fed it with a 1000rms, 600 is no problem. As long as the gains are set properly which i mentioned in the first post) and an orion hcca (one of the most under-rated amps) that is rated at 70rms but pusing out 600. Do you seriously think u can be beaten?
about my experience, ran 450rms to a jl12w3v2 (rated at 300rms) and 800rms to a alpine typeR15 (rated at 500rms).
Im sure u will find a heck load of videos on youtube about under-rated subs, box rise, impedance rise, flexing. Plus there should be a video on my thread if ur youtube works. Do have a look.
@blacklist1 @Talhadon @madhouse12 @CoLd_FuSiOn @Yawar538 @DrShaggy @stj174 can shed some light on this topic as well about sub's being given more power.