Answer:
After seeing an offense by a defender within the circle (or 23 metres area, if serious) and determining that no advantage could be played, the umpire controlling that circle will stop, blow their whistle (tone and length of this depending on the nature of the offense) and signal with both arms horizontal in front of the body, pointing towards the goal.
When the offense stops the ball from going into the goal, however, an umpire will award a penalty stroke.