What is “arrow spur”, and how to use it

The spur will accelerate an arrow into a target and extend the distance it will travel - in other words, increase the performance of the bow from that which would be expected where the limbs were acting on the arrow alone. In essence, the spur is an additional skill an archer can learn to incease control over the arrow power and distance; however, if the archer is lacking in skill of using the technique, or when he begins using the spur, he can compromise the accuracy of the shot.

I see so many online videos of people with bows, shooting an arrow, the arrow leaves the bow, and then the limb tips of the bow fly forward, the archer spins the bow right-way again and nocks another arrow. Many online demonstrations are highly mechanical ways of attempting a "spur" - the experience of the bowman is limited. I can't say this for sure as seeing every one their shots and it's accuracy is not possible. 

Doing the spur well can initially be a difficult manouver to exert on your shaft; however in time, it can be another tool in your box you collectively use every day. It takes many failed shots to aquire the feel, the perfect balance of forces needed to be appied to you bow hand, to shoot the shaft true, fast, AND to strike your mark.

Once the string leaves you draw fingers your bow hand must take full controle to keep the shaft true; you wont have this however, untill you practice the feeling to move that arrow to its target, into your being. 

Thus, the arrow spur can be best understood if you pick up a slow 30# bow. Relese the string with the nocked arrow and feel the weight of the arrow. The goal is to push the arrow into the target without letting the fletchings strike the stave. Relese the string and feel the powerpoint as it pushes a shaft. Typically you would not want to pre load the bow before releasing. Power is minimally placed into the stave during the shot cycle. 

Wait until the arrow is shot before power is applied to rotate the bow forward.

The spur is an adaptive movement and based on the requirment of the shot. Generally, the bow hand movement is a forward push and a rotational "lift up" up of the limbs - but the arrow MUST stay true through its flight. Practice by taking long shots and note the flight of your arrow.

Adding too much rotation will deplete the power to you arrow.

You also must remember the slight roatstion sideways to the stave to achive a liniar movement for the bracing string, this is a "khatra" rotation which you should already be comfortable with as an archer who shoots a shefless bow.

If the arrow fletching is cutting you, you are doing the spurr wrong and must change your hand movement timing. You aren't "feeling" the arrow. Use a thumb gard that is thin - a sting will tell you that shot was wrong.

Feel the requisite counter torque on the bow - but never add so much you ruin the trajectory of you arrow.

You should get to the point where the spur no longer is something you "do" or achieve, but just implement naturally to ensure that a given arrow reaches or gets driven into the target.

In any shot however, dont think of the bow, only the shaft.