Drop through decks are mounted under your trucks. Drop through decks have two cut outs in the nose and tail of the board, and thats why they call them drop throughs. When you assemble your board you have to take apart the trucks, then mount the base plate of the truck onto the deck. After putting the base plates on you can put your truck together and then put your wheels on.
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You can also get shock risers to put on the under side of your deck to obsorb any pressure on the deck. This reduces the chances of getting pressure cracks, which will shorten the life of your board.