It depends on which processor you're asking about, intel spec finder has the details on this. My 920 offers +2 if one core is active, +1 if two to four cores are active. The +2 is pretty difficult to achieve as it's rare to have three cores completely idle.
If the processor runs too hot, draws too much current, or uses too much power, it puts the multiplier back down. These limits are intel specified, and I'm yet to find any information on when they kick in. Anecdotal evidence puts it at around 1.35V, 4.2ghz . When it throttles, as it's too hot etc, it goes back down to non-turbo highest multiplier. It switches between turbo and not turbo many times a second, someone on XS wrote a program to monitor this. If it's far too hot or using far too much power it'll throttle back to stock multi and stay there.
Most motherboards have an option to override this throttling effect, though it may be vaguely described, while maintaining the higher multiplier. The overall effect is like being bribed into using speedstep, with an overriding bios option that makes it behave as you'd probably want it to.
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