Steve Rogerson wrote re Spike's chip:

Wrong. When Druscilla makes her brief return, she shows him that the chip's power is all in his mind and he can overcome it with willpower. He does just that and kills a human.

Okay, a bit off topic, but as a Spike redemptionista (see http://www.bloodyawfulpoet.com for details if desired) I feel the need to correct this. Spike did _not_ feed off a live human being in 'Crush'; Drusilla snapped the girl's neck before passing her to Spike, at which point he hesitated before deciding to drink from the dead girl. There _is_ some confusion over this in Buffy fandom, but the shooting script makes it quite clear that the girl was already dead at _Drusilla's_ hand.

Spike's chip, however, does function on intent to harm (although that may be one factor among many). In the episode 'Fool for Love' (the Spike origins episode, prior to 'Crush'), Spike is able to spar with Buffy, because he has no intent to harm her, and therefore the chip doesn't kick in.

There is at least one piece of evidence in B7 that Gan's limiter may similarly function on intent, or more accurately, on emotion; Blake says in 'Breakdown': "The limiter is supposed to cut in when stress drives him to the point where he might kill." So the trigger appears to be rage, not killing per se. This would explain why he's able to indulge in some self-defensive actions in 'Deliverance' (and possibly 'Cygnus Alpha' as well), because he isn't experiencing sufficient rage to trigger the limiter.

Another possible explanation is that the limiter had already begun to malfunction - Blake also says in 'Breakdown' that it had been going bad for some time.

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