Contrary to popular culture via Hollywood, when a car drives over a fire hydrant and breaks it off, water does not spray in the air like they show on tv.
They have break-away devices that allow it to be broken off near ground level without the water spraying in the air like that.
The knob on the top operates a plunger type device at the base of the unit that is typically about 3' below ground.
Sometimes you'll see paint colors at the top that isn't the same as one say... a couple blocks away etc. That is a color coded flow rate that shows the firemen how much draw the pump truck can place on a hydrant. See if it's a low flow system the pumper has to suck water out to keep hoses pressurized. But too much suction can collapse certain pipes or suck gaskets out of the joints. Very bad, and very expensive to fix later.
I'm not a fireman, nor play one on tv. I'm a consultant roadway inspector and that involves all aspects of infrastructure construction or upkeep. Fire hydrants are one of my favorite items.
Once I rejected some about to be installed. The "brand" was not on an approved list of allowable due to a weak brass fitting in the break away part. This lead to the part breaking while fire fighters were hurredly cranking this brand open during fire calls. It was a contraversial call on my part. Meetings ensued. A batch of folks from the company, engineers, repair folks and firemen attended one. An engineer says "a fire hydrant is a fire hydrant, who cares?".... A fireman retorted "say that when your house is on fire and that coupling breaks so I can't get water to put the fire in your house."
A few weeks later the manufacturer redesigned the coupling and by late that year those fire hydrants were placed back on the list of approved fire hydrants in that city.