The 'magic knot' you make a small loop in the line then take the line and tuck another loop up through it creating a fixed loop in the line. It serves the purpose as the lower block in a block and tackle system an when you now take the tail of the line through the tie down ring then back down and through that loop to go back up, you have created a 'purchase' adding leverage then I do that one more time.
The advantage of making the purchase loop with that knot is that no matter how tight it gets tied, it can be removed again by pulling both ends of the line. If you use an overhand knot to create the purchase and pull it tight, it'll take a knife of axe to get it out, especially if it gets wet. The magic knot may get tight enough that you can't pull it out by hand, but tie one end to a tree and the other to a truck, and regardless how tight it is, it comes out.