It will not fire off-skin.
Sill holds the lamp until the flash window is seated flat on skin. Lift the head mid-pass and the flash stops, so the light stays on the area you are treating and off everything else.
The eye is the reason.
An IPL pulse is bright enough to matter to your eyes at close range. Safety glasses come in the box and you should wear them for every session. Sill is the second layer: with no skin under the window, there is no pulse to shield against.
It also settles the question people ask before their first session, which is what happens if the device goes off in their hand. It does not. The window has to be in full contact before the lamp will fire at all.
What Sill reads is contact, not colour. Nothing in the Pearl 01 reads your skin tone or caps flash energy according to it. You set the level yourself and the number on the display is the number you get. If you want the machine making that decision for you, buy a device with a tone sensor.
No wasted flashes.
Every flash off-skin is a flash off the lamp’s rated life and a patch you have to go back over. Contact-gating means the count on the lamp is a count of pulses that actually landed.
On curved areas, the knee and the ankle and the jawline, it also tells you when your angle is wrong. If the lamp goes quiet, the window has lifted. Flatten the head and it picks straight back up.