When you panic stop, you may initially hear a single chirp. Since the Boxster is so nicely balanced, the front wheel(s) momentarily break free in a panic stop. All that load you're piling onto the front suspension while decelerating pushes the tires more firmly into the pavement, causing the skid to stop. This is why you try on the track to load your brakes progressively instead of stabbing at the brake pedal - you want to shift load to the front suspension before threshold braking.
There's a spot on my way home where I love to chirp the front brakes during threshold braking. I had not realized until today that this is cause for embarrassment.
