This is a Sunday Knot. It is used to make a lope out of wire rope or cable with out putting any kinks in the wire. You can use this for regular rope as well.
Think of this as putting you right hand up your left sleeve and your left hand up your right sleeve.
This knot is often used to create the loop that you need to create a Prusik Knot; shown below.
This is another pressure knot used to attach sandbags to rope lines. It is most often made of thin wire rope (cable); however, now that new synthetic rope is being made that is stronger then cable (and won't kink) we are seeing other materials being used.
Remember that this is a loop.
The loop wraps around the rope 2 times.
After the second wrap, pull the other end of the loop through.
Pull snug. When you put weight on this loop, the knot will get tighter.
