It's theoretically simple, but because the bow is held at an angle (about 45 degrees) there are lots of weird angles.
Something that I screwed up with the mk1 is that I forgot to place the trigger mechanism up above the bow mounting (as in, the arrow passed over your hand... not through it!).
It also needs to have the main spine of the device set away from the bow and release mechanism to avoid the string slapping it (like it can on your arm). This is also why the track stops about a foot from the bow mount.
To stop the release mechanism sliding back when you are trying to cock the bow there is a magnet to hold it.
I made a wingnut spanner from a scrap of ply so that the A frame ( which fold up of course) can be tightened up solidly (bottom pic)
Care is taken to arrange the pulley so that it pulls dead in line with the release mechanism. Also the stirrup is placed so that pulling on the rope tends to force the front of the device down onto the ground rather than lifting it up.
I found it did flex a bit in use, so I added several blocks of plywood, glued in to stop flex and twist, these were just done on an ad hoc basis with a bit of quirky artistic interpretation to give it a steam punk / art deco look!
The main spine is a bit of 5 x 3/4 from an old bed frame.
The track is a slab of 3/4 ply for stability which is screwed on the the spine. The other bits are odds and ends of ply glued and screwed on to line up appropriately.
Enough chat, here are the pics.
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