Many cocking devices used in combination with the simple medieval nut and trigger lock - for example cranequins, winches, goat's foot levers and wippes - push the rear end of the nut backwards with the bowstring, rotating it until the trigger catches the notch of the nut. After this the crossbow is ready to shoot. Usually, the area behind nut's fingers is flat, like here:
This means that the cocking device has to pull (or push) the bowstring way past the centerline of the nut. This obviously limits safe draw length of the crossbow, given same safety margins. The shorter the draw length of the crossbow, the bigger the problem. In the above picture the problem is made worse by the fingers being far away from the centerline of the nut, which is generally a good idea.
Nut with a quick-locker pin Edit
There is fortunately a few simple solutions to this problem. The simplest is to make the rear-end of the nut slope upwards, so that the bowstring rotates it quicker. An alternative solution is to add a special pin to the nut, which catches the bowstring as soon as it has passed over the fingers. From the side, slightly elevated:
From the side:
And from the above:
If the pin is properly aligned, the bowstring does not have to be pulled at all past the cocked position.