it is fairly easy to understand. when your limb tip flexes, it moves in an arc as the arrow is pulled straight back in the center. That means the tip will have a horizontal component and a vertical component, that will eventually send the arrow forward. the farther back you pull the string, the greater percentage of the tip motion will be vertical instead of horizontal, and the greater the angle of the string to the limb will become. If you could imagine what would happen if you pulled the string back completely, until it was perfectly doubled up (an impossibility, as the bow would also have to be folded in half) you would have a string angle to the limb of 180 degrees (a straight line), and you would not be able to draw farther, no matter how hard you pull.
In reality, the bow hits a limit much earlier: as that angle passes 90 degrees, the vertical component starts to overpower the horizontal component and you get 'stack'. The draw weight starts to climb quickly as more and more force goes to squeezing the bow in half instead of pulling the tips (and the arrow) back. This is one reason to recurve the tips, as it delays the point where string angle with the tip exceeds 90 degrees and allows you to pull farther. See for example the very short asian composite bows with extreme recurved tips
