想了一天还是看不懂的一句英文，请各位大神指教

Risk of loss of Products sold to Distributor by Seller shall pass upon proper loading of the Products on the truck, plane, train or ship of the common carrier by Seller.
请问“upon proper loading of the Products ”中1. upon的用法是什么？是否表示在某一特定情况下？
2.“loading of the Products ”of 能否省去？