Fisker did a road-show with the almost-production-ready car in the summer of last year. The biggest, and most common complaint about the car from the participants and the press was that the ICE engine was extremely loud and discordant, and was not in keeping with a six-figure car. I drove one of these cars (you can find my post about it if you search) and I agree. The car was very loud.
To counter this, Fisker made a number of changes to the cars before production release. The most notable was the re-engineering of the muffler and sound-supression system, and the installation of the "pizza box" pre-muffler. They also made changes to how the ICE operated, including changing the correlation between throttle position and how frequently and hard the ICE operates. This last item is what's relevant here. The ICE in sport mode now lets the battery deplete to 50% (about 26 miles range) and recharges the battery only sufficient to then maintain this level. In the pre-prod cars, the ICE would actually increase your stealth-only range, but in the interest of having the ICE work less hard (and more quietly) this approach was changed.
Edit: As mentioned below, this also reduces the amount of emissions the car produces.
The car now effectively has three driving modes (ignoring the various Hill mode multipliers):
1) Stealth (battery only)
2) "Battery Depleting" Sport Mode (The ICE runs and the battery is discharged until it's level of charge is approximately 26 miles. Full power is available to the electric traction motors)
3) "Battery Sustaining" Sport Mode (The ICE runs and the level of charge of the battery is maintained, but does not increase. Vehicle is propelled only by electricity generated by the ICE)
Note that if you run your car in Stealth mode and the battery runs out, the car will automatically kick on the ICE and you'll essentially be in "Stage 3" above, even though the car still indicates "Stealth" on the dashboard.
I think the video above describes the pre-production behavior.
BTW, if you poke around the forum you'll find some tricks for how to use Hill mode to actually increase your battery range from less than 26 to 26 while you drive.
Brent