(a) When do you choose to throw? Before or after the dice are rolled? Before or after the opponents action is revealed?
(b) Does the axe really work to turn any independent test into a versus or is that a misprint and it should just be attacks like the spear? If not, how does that work with what was originally an independent test where only one side was rolling?
a. As Storapan said, before dice are rolled but after actions are revealed.
b. Essentially, it’s a defense against an Attack against Attack situation. Normally these are independent and very bad! But a hurled hand axe protects against that!