I've never let go of the handles when its freewheeling. I have noticed when you push the handles all the way forward it freewheels more often, and it catches when you pull back. I've pulled back hard when going downhill.
Maybe it's not really free wheeling but while going down hill it's going as fast as the hydraulics will allow it to move. If you try going down the hill slowly does it still try to run away or can you control the speed?