Here is the "soft launch" definition from Wikipedia; of course you don't completely trust Wikipedia, but it's a first start, especially when you credit it.
Soft launching is the method of launching a missile (such as an anti-tank guided missile) in such a way that the rocket motor ignites outside of the launch tube; the missile is ejected non-explosively. The point is to minimize the risk of damage to the launcher by maintaining a safe distance. Contrast this with hard launching. A similar concept called cold launch was pioneered by the Soviets for application in vertical launch system clusters on board ships.
Soft launching lets you tweak and revise. You get the word out there and you gauge interest. You know what works and what doesn’t.
Plus, you get to make mistakes while you’re still in the shadows. Messing up in front of a smaller crowd means you’ll be better off when the bright lights eventually do shine upon you.
The hype cycle now lasts less than a day. Take yesterday’s over-hyped launch of stealth search startup Cuil, which was quickly followed by a backlash when everyone realized that it was selling a bill of goods. This was entirely the company’s own fault. It pre-briefed every blogger and tech journalist on the planet, but didn’t allow anyone to actually test the search engine before the launch.