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I generally don't back a clove hitch up because you want the tails directed away from whatever it's hitched to for maximum strength and minimum slippage. If you half hitch it you're potentially feeding a tail into the knot which may slip a bit easier... I'm just guessing though, never done it that way.
Just because it doesn't LOOK secure doesn't mean it isn't... it works exactly how a knot is supposed to by relying on the rope's friction against itself.
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If you use a clove hitch for lowering a limb the clove hitch <u>must</u> be backed up with two half hitches. If not, there is the chance that the clove will 'unroll'.
Try this--tie a clove hitch on the ground on a smaller (6-14" diameter) tree. Then hold the standing part and walk around the tree against the clove hitch. The hitch will simply unravel all the way to the end of the line.
But, if you tie a clove on that same tree and then pull in one single direction the clove will hold all day long.
In arborist rigging, it is recommeded to always backup a clove hitch with two half hitches because the piece can swing, spin, and unravel.