The D- loop is used to nock your arrow.It provides a place to attach your hand release instead of the actual bows string.Preventing while at full draw string twist and tourking the handle on the bow,causing side to side twisting while at full draw.
This saves on string wear and arrow pich while drawing your bow back at full draw.It just keeps everything in its original position at set up on the bow by adding a D-Loop to your bow string.It also helps with diff. folks while anchoring at full draw no matter where they like to anchor because the D-Loop will turn some to allow this.It will also give you some added arrow speed because you really don't need a" brass nock" on the bow string.Anything you add to a bows string will slow down its arrow speed.It also will help you with bows of today with their shorter ATA...THE length of the bow from axle to axle !The shorter the bow the more string arch and "V" SHAPE the string will be,so adding the D-Loop will help pervent this.It also will add from 1/4" to 1/2" of draw length to your bow if you need it.
Just good safe insurance! I like it in a big way because if you tie a D-Loop right with both knots opposing each other it keeps your Peep sight always in its correct place on the bows string with drawing it back everytime. No need for the rubber tubing this way?
If you can copy the pic and make it bigger so as to see it better.you tie in your peep sight using the video i show above and when you in stall the D-Loop you line up the peep sight and the D-Loop then while its all lined up you really pull hard on the center D-Loop with your shooting release.pull very hard.Then sight down the string and make sure your Peep and D-Loop are both in line with each other.the opposing ends of the D-Loop knot will keep the string from twisting and turning on you.your peep will always line up.