When an object is in an elliptical orbit, the tidal forces acting on it are stronger near periapsis than near apoapsis.
Sustained tidal heating occurs when the elliptical orbit is prevented from circularizing due to additional gravitational forces from other bodies that keep tugging the object back into an elliptical orbit.
Io's eccentricity persists as the result of its orbital resonances with the Galilean moons Europa and Ganymede.
[1] The same mechanism has provided the energy to melt the lower layers of the ice surrounding the rocky mantle of Jupiter's next-closest large moon, Europa.
However, the heating of the latter is weaker, because of reduced flexing—Europa has half Io's orbital frequency and a 14% smaller radius; also, while Europa's orbit is about twice as eccentric as Io's, tidal force falls off with the cube of distance and is only a quarter as strong at Europa.
Jupiter maintains the moons' orbits via tides they raise on it and thus its rotational energy ultimately powers the system.
[1] Saturn's moon Enceladus is similarly thought to have a liquid water ocean beneath its icy crust, due to tidal heating related to its resonance with Dione.
The water vapor geysers which eject material from Enceladus are thought to be powered by friction generated within its interior.
[3] Egbert & Ray (2001) confirmed that overall estimate, writing "The total amount of tidal energy dissipated in the Earth-Moon-Sun system is now well-determined.
The methods of space geodesy—altimetry, satellite laser ranging, lunar laser ranging—have converged to 3.7 TW ..."[4] Heller et al. (2021) estimated that shortly after the Moon was formed, when the Moon orbited 10-15 times closer to Earth than it does now, tidal heating might have contributed ~10 W/m2 of heating over perhaps 100 million years, and that this could have accounted for a temperature increase of up to 5°C on the early Earth.
[5][6] Harada et al. (2014) proposed that tidal heating may have created a molten layer at the core-mantle boundary within Earth's Moon.
represents the imaginary portion of the second-order Love number which measures the efficiency at which the satellite dissipates tidal energy into frictional heat.
[9][10] The rheological parameters' values, in turn, depend upon the temperature and the concentration of partial melt in the body's interior.