This one has always tied me up in knots as well. I would say that the answer was -2
In the manual just below the section about b_leap is b_growth, which gives an example:
b_growth for the price change from 4.0 to 4.2 is 0.2. If the price changed from 4.2 to 4.0, then it's -0.2.
To my mind, that indicates that if a price is coming in (or steaming or the odds getting shorter or whatever way you want to describe it), then b_leap, like b_growth, is a negative number.
Just my interpretation, not saying it's correct.