system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a compound bow work, particularly the function of the cams and the "let off"?
gpt
Because a compound bow differs from a recurve bow in that it has shorter limbs (the part that extends past the grip), the power to drive the arrow forward into flight comes from a system of pulleys, or "cams" on the ends of the limbs.


Essentially, a regular recurve bow works by putting force into the limbs by pulling back on a string.  The force is transferred to the limbs so that they are drawn back, and when the string is released, the limbs snap forward, pulling the string and the arrow along with it.  This produces the force that propels the arrow.

Compound bows, on the other hand, instead of putting the force of the string at a perpendicular angle to the limbs, use their pulleys (cams) to essentially try to pull the limbs together instead of back during draw.  There is the part of the string that the arrow rests on, but there is an additional length that is doing the actual "pulling", and it pulls straight up and down on the limbs, pulling them together.  When the string is released, the limbs snap back both up and down, instead of forward, making it easier to control. This reduces the strain on the archer, and allows for higher accuracy in a more compact frame, since you're not fighting against the bow trying to fly forward out of your hand.  this design comes with the trade-off of needing stronger materials to function properly, but that isn't much of an issue with modern materials.


As for "let-off", the cams can be designed and positioned in such a way that the strain of holding the bow at full draw (all the way back) is reduced by a certain percentage, usually anywhere from 70-85%.  If you look at a modern compound bow, the cams are not circular, but rather a strange ovoid shape; this is the design that allows for let-off.  


Let-off allows trophy hunters or casual archers to hold a drawn shot for longer while they sight the target or wait for their prey to come into range.  If the let-off weren't there, you would have to hold a full 80 lbs or so of draw, which is difficult to do for any length of time; a let-off of 80 means that you're dropping 80% of full draw weight at the very end of your draw, so you only have to hold 20% of what you did to draw it back.  An 80lb bow with a let-off of 80 only takes 16lbs to hold at full draw, which can be done relatively easily.