Our eyes are never completely still, even when we focus on a small target. Small movements of drift and minute darting about called micro-saccades occur.
With the first step used to make the corneal flap, the eye is held in place with a plastic ring (fixation device) positioned over the white of the eye, with suction holding it in place, minimising your ability to move during this delicate part of the procedure.
The Wavelight EX500 Excimer laser uses iris recognition and an infrared eye-tracker which checks the eye position over 1000 times per second. It is capable of following any small micro-saccades and actively adjusting the laser beam to a new position as those small movements are detected. This provides excellent safety and computer control.
If excessive movement occurs, the laser stops and can be restarted once you find the fixation target light again.