On a safety kick the restraining line for the receiving team is the 45-yard line. The restraining line for the kicking team is the 35-yard line. That's 20 yards away instead of the normal 10 yards away on a kickoff.
PRO TIP: Not many football fans know what a safety kick is and how it's different from a normal kickoff. It looks the same, but if you look carefully, the players line up on the field differently.
A safety happens when a team retreats into their own end zone with the ball and makes a mistake. The opposing team gets two points for this mistake. It's the only way the team on defense can score points in football.