How Does an Uninterruptible Power Supply Work?

Cringe.

I believe that you may refer to an extension cord.  They have been on the market for awhile, I believe.

Standby power systems (in Britain sometimes called "uninterruptible") operate by these means:

Gasoline-diesel-LPG systems are constantly in the standby mode, and when the line meter senses phase outage, or power interruption, it will go through a "takeover" procedue.  It will disconnect the resident system from the power supply line, start its engine, bring the generator to phase, and engage the generator to power the resident system.  When the line power returns and is stable, the standby generator reverses the takeover in a "release" procedure.

Battery systems charge very large storage batteries from the power input and standby.  When the line voltage or phase becomes unstable or drops, it operates as the diesel system above except that the batteries power an inverter, which converts DC to AC at whatever voltage and phase relationship you need.  The takeover and release procedures are the same.

Antiquated "flywheel" systems used a heavy flywheel to maintain power to the service panel, and the utility power merely kept the motor turning the flywheel which was in turn hooked to a generator that supplied the panel.

The only advantage was that if the power to the panel was discontinued the flywheel stored enough energy to supply your panel while the backup generator started and came up to phase and voltage.  Battery systems do much the same task.  They are much more efficient, cost effective, and safer.