I don't know exactly which move (number) are we talking about. I tried to explain how I understand the rules.

Let's say, that there is

(a) a black piece on c1, white on c2 and nothing on c3 = black can't jump, because he can't move backward at the beginnig of his move...

(b) a black piece on a1, white pieces on b1 and c2, and c1 and c3 are empty = black can/must jump from a1 to c1 and because he's already moving, he must jump further to c3.