With a typical xenon flash, TTL compensation usually changes the total flash energy, not by making the tube glow “brighter” in a simple continuous way, but mainly by changing how long current is allowed to flow through the tube.

In most speedlights, a capacitor is charged before the shot. When the flash fires, that stored energy is discharged through ionized xenon gas in the tube. At full output, the flash is allowed to discharge for longer; at lower output, the electronics cut the discharge off sooner. So +EV compensation tells TTL to deliver more light, usually by allowing a longer flash duration, while -EV cuts it shorter.

As a rough guide, full power may be around 1/1000 s, while low power can be much shorter, such as around 1/25,000 s.

So in practical terms: TTL flash compensation usually changes flash duration (and therefore total energy delivered), rather than adjusting a simple wattage/brightness control.