Yes that is it.............they work like a ladder lock on a backpacks shoulder strap. As weight is applied to the hammock or buckle the outer ring squeezes the strap between the sliding bar and the inner edge of the outer ring. The slider bar usually has a ribbed or tooth section that bites into the strapcoming fromt eh tree locking it into place.
Easier example would be a Chinese Finger Trap.............the harder you pull your fingers apart the more the trap squeezes on you fingers.
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