Low Water Level
Disclaimer: I am not an engineer, this article is constructed from the advice of many people I know.
When a low water condition is noticed (nothing in the gauge glass) the operator should not draw or pull the fire. You cannot safely remove the fire through the little bitty firebox door in a hurry when there is a low water condition.
If the water is indeed low, and you are moving, the sloshing will keep the sheet from getting overheated. When the condition is noticed, it should be double checked with the trycocks in case the gauge glass or water column is plugged, or the glass is completely full.
The moving engine should be backed into a hole, to get the back end down, so water covers the crown sheet. If it does, no worries. If not keep reading. If stationary, you need to smother the fire by covering or plugging the smokestack, and closing all sources of draft. If no oxygen gets in, no fire burns, and no further heat is made. Stopping the products of combustion from getting out does this as effectively as stopping off the draft. Doing both is insurance, but the best thing to do.
Poking about in the firebox stirs the fire up, making it hotter, and exacerbating an already bad situation, perhaps from bad to blinding when your head is in the firebox door and the fusible plug melts, blowing live steam, and burning wood/coal and ashes out the door.
The term "pulling the fire" applies to stationary plant operation when pulling the fire involves turning off a valve. I questioned the possibility that sloshing water onto the hot crown sheet might cause an explosion and James offered some further information in a later message to clarify what he said and help my understanding - I've included this later information here as other people may have misunderstood in the same way I did, and it is vitally important that this particular aspect of steam boiler operation is done right every single time:
If you are moving, and there is no water showing in the gauge glass then the sloshing of the water will keep the crown sheet from getting red hot .....etc. If you are stationary, this is a no deal at all. Stationary engines working with a low water condition, plugged fusible plug, and hard fire should not be moved AT ALL. The fire should be immediately smothered, no engine movement at all is allowed, or what you describe can happen.
BLEVE or Boiling Liquid Expanding Vapour Explosion is no school of thought or theory. Movement or non movement does to an extent dictate what you should do. If you are unsure, just make the fire stop producing heat by smothering it, go away and let it cool, then inspect for damage later. You might want to counsel others to stay away as well.... Boiler explosions are rare, but historically, in the majority of cases were caused by low water conditions. Mechanical failure from over pressure due to pressure reducing valve failure would probably be second, but I have no data on that.