Basically the work out the odds they think it should be and check it against the bookies. If they think the bookies are wrong and the odds are to high they advise clients to put money on it. I believe they are paid for advice only although I'm sure they follow bet themselves as it's clearly...
In basics I believe Asian handicap for example will have the underdog in the lead. For example the underdog in the Sunderland v man Utd game may have been Sunderland and they are given a 2 goal head start. You can then bet on the outcome. They then employ some maths nuts from God knows where who...