The faint extended broad (>=1000 km s-1) optical emission lines associated with giant H II regions are shown here to be produced in a shell of ISM material smoothly accelerated soon after breakout. Two-dimensional calculations of remnants caused by a strong energy deposit in a low metal abundance ISM are here shown to undergo breakout once encountering a steep density gradient, leading to a fast-moving shell capable of producing the broad and faint emission lines. Energetic sources lead to fast, thick, and hot shells, and when evolving in a low-metallicity ISM, to quasi-adiabatic shells that strongly delay their fragmentation owing to Rayleigh-Taylor instabilities. At the same time, these are smoothly accelerated to reach large distances from the breakout point. The shell acceleration is promoted by the passage of several shocks with small relative speeds, caused by the continuous push exerted by the hot gas that steadily increases its speed to fill the deformed superbubble volume.