As a stone thrower lobs its shots in a high arc, it can be used to attack enemies that are hidden from its view. Such shots are inaccurate at best, as the stone thrower's crew essentially have to guess where their foe lies, working from fragmented and often unreliable information provided by other warriors.
If you wish to fire indirectly, declare that you will do so before taking the stone thrower's shot. An indirect shot does not require line of sight, but is otherwise treated as a normal shot and has the usual requirements (the target needs to be in range and outside the minimum range of the stone thrower).
An indirect shot is not as accurate as one where the crew can see their target. If an arrow is rolled on the scatter dice, resolve the scatter as normal. If however, you roll a Hit! on the scatter dice, the shot does not land on target (as would normally be the case) but merely scatters less – how much less depends greatly on the skill of the crew. You'll notice the Hit! symbol has a small arrow at the top of it – this indicates the direction of scatter for an indirect shot – the distance travelled is equal to the score shown on the artillery dice minus the crew's Ballistic Skill characteristic (to a minimum of zero!).
Other than the aforementioned inaccuracy of the scatter, an indirect shot from a stone thrower is treated exactly according to the rules given for normal shots given above.