Pringles is right. Amps "pushed" past their maximum will clip the top and bottom of the sine wave. This introduces a dc signal into the speaker/sub, which then forces the speaker into maximum excursion. Done for long enough, will blow ur speaker.
"Underpowering" speakers is a term which is used too losely and is actually just wrong. A speaker will NEVER blow if u underpower it. The reason "underpowered" speakers blow, is because people who dont have big enough amps or gains, and want louder music, turn the head unit to maximum. Head units are good upto a point, but like all amplifiers, have limits. In most cases, past 75 percent maximum volume, the internal amplification of the head unit tends to bring noise and distortion. This is then sent to the amp, which, when amplified further, brings spikes to the speakers. These spikes will sound like crackling, or just crappy sound in general. Underpower a speaker will never kill it, just trying to make underpowered speakers loud, will.
A good rule of thumb, the more expensive the head unit the better internal amplification. 75% max volume is a good limit to set, for expensive units, its more around 85%. Never take a head unit to Max volume.... ur begging for ur speakers to blow... and depending on ur setup... Your ears too!