This invention relates to archery bows, and particularly to compound bows. 
A major advance in archery bows is the compound bow, disclosed in Allen U.S. Pat.  No.  3,486,495.  The compound bow, comprising a system of metal cables and eccentric wheels attached to the limbs of the bow, makes it possible to store a greater amount of energy in the drawn bow than is possible in conventional bows requiring the same maximum pulling force to reach the fully drawn position, yet requires less pulling force to hold the bow in the fully drawn condition.  Because the pulling force required at the fully drawn position is less than the maximum pull weight, a lighter, more flexible arrow may be used with a compound bow than with a conventional bow having the same pull weight.  The compound bow, therefore, provides higher arrow initial velocity and a flatter trajectory over a given range, allowing greater accuracy. 
The sport of hunting game with bows of all types requires great stealth and skill in order to approach the quarry within the short range capability of the weapon, draw the bow, and aim shoot an arrow before the bird or animal is aware of the bowhunter's presence.  Any movement by the bowhunter must be slow and deliberate, in order not to attract the attention of the quarry.  Once within range, the hunter nocks an arrow on the bowstring and draws his bow, preferably while his quarry has not yet noticed him.  If the animal notices the hunter, the hunter must instantly choose between continuing to full draw and releasing the arrow, thus making a hasty shot, and attempting to remain motionless with the bow at partial draw.  The great difficulty with the latter alternative is that the hunter's arm and shoulder muscles quickly begin to fatigue and tremble while opposing the force of the partially drawn bow, causing both an unsteady aim and movement that is likely to be noticed by the animal. 
This problem is more exaggerated in a compound bow than in a conventional bow.  As the compound bow is drawn, the eccentric wheels are rotated by the cables, causing the force required to draw the bow to rapidly increase during the early stage of drawing the bow, reaching a point of peak pull somewhat short of the fully drawn position of the bow.  As the bow is drawn further, to the fully drawn position, the pulling force required thereafter gradually decreases.  In contrast, the pull of a conventional bow increases nearly linearly with the distance through which the bow has been drawn.  As a result, except within a small final portion of the draw distance, the user of a compound bow, in holding his bow in a partially drawn position, must oppose a force which is greater than that of a conventional bow of similar pull weight.  Even further exaggerating this problem, the reduced pull force of a fully drawn compound bow allows accurate use of a compound bow whose peak pull is greater than the maximum pull of the strongest conventional bow which a given person could accurately use. 
One attempted solution to this problem is the bowstring release device disclosed in Barrick U.S. Pat.  No.  4,041,925, which holds the bow in a fully drawn condition.  The Barrick device, however, is incapable of holding the bow partially drawn and allowing continuation toward a full draw.  Additionally, use of such a device is illegal in many states.  Such devices also have the drawbacks of being too cumbersome for use in hunting and of tending to interfere with the cables of compound bows. 
So as to avoid having to nock an arrow on the somewhat hard and inelastic cable which passes around the eccentric wheels, most compound bows include a bowstring of softer material, fastened usually by hooks to two separate lengths of cable which pass around the respective eccentric wheels.  Because of the mechanical advantage provided by the cable and eccentric wheel arrangement, the limbs of a compound bow are very stiff, making it very difficult to flex the bow to replace such a bowstring.  Restringing a compound bow may also require the use of special tools and a series of adjustments to return the bow to its properly tuned shooting condition.  While some compound bows are equipped with a double set of bowstring attachment points on the cables, allowing installation of a new bowstring while the old bowstring remains attached, even this arrangement requires some outside means of maintaining tension on the old bowstring while installing a new bowstring on the second pair of attachment points. 
Since shooting accuracy depends on skillful familiarity with the characteristics of the particular bow in its properly adjusted condition, for best hunting accuracy it is preferable to leave a compound bow strung and adjusted during travel to a hunting locality.  Some states, however, prohibit transportation of a bow which is ready to shoot.  A device is needed, therefore, which can disable a compound bow without removal of the bowstring, to allow its transportation without need for extensive adjustment prior to use. 
What is needed, therefore, is a device capable of allowing a bowhunter using a compound bow to relax his tension on the bowstring while the bow is partially drawn without making a movement which is likely to be noticed by the game, which will facilitate replacement of the bowstring, and which is capable of disabling the bow without removal of the bowstring.