Jonny that can happen after the cast but if a fish makes off with the lead/feeder it quickly disengages because of the shallow gape of the hook. I have yet to land a fish where the hook has not disengagaed but have reeled in from a cast without it having release.
If the lead becomes snagged with the hook still engaged you simply bounce your rod against the lead a couple of times to release the hook.
Woody, the purpose of the rig as shown was to allow the lead/feeder to be shed from a running rig when fishing snaggy waters such as the ribble.
Jason, it is a variation of the rotten bottom rig used in sea fishing. The major difference apart from it being scaled down is the use of the powergum, bead and float stop as the weak link. This link will release the lead under slow pressure as well as a sudden increase in pressure. It does so much more smoothly than using line and doesn't have to keep being replaced unlike 1 1/2 lb line.
Alan, if you make up the rig using the links i use and test the strength of the hook you will be amazed. for casting 3 oz it is safe.
One thing to note is that if you forget to attahc the hook before you cast (as i have done quite a few times) the lead/feeder will come off but it will have enough forward motion to make it travel 10yards or so. Annoying when you do it but safe.
If you tie up the rig and use it you will be amazed at how simple and effective it is. It has the added bonus that you can change the lead/feeder without retying your rig.