When it was dependent on how many pax and drivers had the app open for use in an area.
When you could see the cloud, go offline and drive to it (only if close!), turn on the app when the color was deep enough (or before it lightened too much for you) and get a ride at or close to the rate showing on the map.
When surge was used as a price negotiation between riders and drivers. What's the highest rate the pax was willing to pay, and what's the lowest the driver is willing to accept.
When the pax was the driver's customer, and Uber was just the facilitator connecting them, and taking a fee for doing so.
That was real surge. Back before all this upfront pricing nonsense designed so Uber could skim instead of taking a fair percentage.