The Baby Jogger Summit X3 has both a parking brake AND a control brake on both rear wheels, plus a safety handle wrist strap.
Both rear wheels have drum brakes operated by a lever mounted on the handlebar.
squeeze the lever to apply the brake
The Control Brake can be useful to slow down the pushchair if you are moving at speed or help maintain control on a steep slope.
Rear Wheel Parking Brake
Both of the rear wheels have a brake cog into which a metal rod slides to prevent them from turning. The metal rod is operated by a brake lever
push the lever down to engage the brake
pull up to disengage
Note that the brake rods are sprung loaded so that if they are not initially aligned with the brake cog they will instantly engage with any rotation.
Safety Wrist Strap
A safety wrist strap is attached to the handlebar, always loop your wrist through the strap before setting off, even if you are walking on flat level ground - if you always do it then you will never forget. Should you stumble and let go of the handlebar the wrist strap prevents the pushchair from rolling away from you. If you are on a slope and let go of the handle without the wrist strap, then the pushchair can run away from you and very quickly accelerate to a speed you cannot catch up with!