It's presumably capable of supplying up to 425watts if the circumstances demand it without tripping.
I found out a few months ago (after buying a new 500w PSU, that my original Enermax PSU that came with the case I bought six or seven years ago, was only 100w rather than the 235w I had believed all along.
Even so it coped with an amazing variety of components without complaining - the benefit of having a quality product.
The only reason I changed it was because a new graphic card's manual revealed that a 300w (and preferably 350w) PSU was required...:-)
My 26-year-old NAD 7030 tuner/amplifier has a similar feature in that it can deliver up to +3dB overload outputs to the speakers without "clipping".
It's a 30w per channel amplifier but has 60w output transistors to cope with such transient bursts.