Without a door, the leaves of that final pile (which you will pull in over your head) will simply disperse and expose your head (the most important part of your body to keep covered). The door will simply lean against this pile to hold it in place.
What does that mean?
"pull in" means to grasp (the leaves) and pull them towards you. "over your head" means that you place the leaves so that they cover your head.
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"pull in" means to grasp (the leaves) and pull them towards you. "over your head" means that you place the leaves so that they cover your head.