Consider the two cases:
(1) a is equal to b.
Then foo(i) and bar(i) must be invoked in this order. The code does so. Since the return value of bar(i) is the same as that of foo(i), it is sufficient to call bar in a tail call and let it's result return to the caller of foobar.
(2) a is not equal to b.
Then foo(i) and bar(i) must be invoked in this order. The code does so. Since the return value of bar(i) is different from that of foo(i), b, i.e., the return value of bar(i) must be returned to the caller. This can be achieved by a tail call of bar and let it's result return to the caller of foobar.
The two cases do exactly the same thing.
