Set Altitude Mission Planner

When I fly a mission I have the aircraft doing the box pattern around 400 agl. I want to quickly drop the aircraft altitude to say 300 agl and to do this I used the side bar on the main mission planner page. It does descend but it seems like when it moves to the next waypoint it climbs back up to the waypoint altitude. I figured when using this parameter it ignores the waypoint altitudes.