20:57 Dec 1, 2011 

does not refer to the benefits you get when joining the programme. It refers to something else unmentioned
for example you could be told that by joining this programme you receive money off vouchers to the value of £100
this sounds a bit suspicious so you ask "what's the catch" (in other words what's the trick or the thing you aren't telling us to make it sound as if we are getting a good deal) the catch could be that you get 100 x 1 pound vouchers and you can only use one at a time and have to spend 20 pounds before using one - all of a sudden the offer doesn't seem as attractive. This is the catch