Great question! In order to have a leak-resistance experience, it is important to lock the bottle prior to feeds as the incomplete closure of the bottle may allow liquids to escape. This will also be useful when warm milk (breastmilk or powdered milk) is administered as there may be a built-up of air pressure pushing liquids out of the bottle.
To secure the bottle, gently twist the collar/lid in a clockwise direction and you are all set!