Watt-squared-seconds: unit of safe operating area?
When the pulse length is sufficiently short, the die temperature change is approximately constant for \$P^2t\$. So if you know the initial die temperature and the maximum die temperature you can estimate the maximum pulse the die can handle. See the below curve. 



If you calculate \$P^2t \$ for the three points shown, it's around \$300W^2 s\$ for all three. 

This will be derated to 0 for elevated die temperatures. See below from the LTC4233 datasheet: 



You can see the cause of this effect in the transient thermal response curve of MOSFETs: