To fling or throw open (a door, window, etc.); to leave open (so it stays open).
夏は家中の窓を開け放して風を通す。
In summer we throw open all the windows in the house to let the breeze through.
荷物が多くて、つい玄関を開け放してしまった。
There was so much luggage that I ended up leaving the front door open.
店主は冷房を使っている間は勝手口を開け放さないよう注意した。
The shopkeeper warned not to leave the back door open while the air conditioner was running.