Hi Koz!
You can reveal an enchantment between Attack Steps. As long as it does not alter a prior event, it will affect the attack.
For example, if you reveal Rhino Hide immediately after the Roll the Dice Step, it can and will help reduce the damage done, since damage has not been applied yet.
Likewise, Divine Intervention was specifically designed for just such a purpose. Wait for the attack dice to roll, see the result, if it's horrible call upon Asyra to whisk you away before the Apply Damage and Effects Step! Watch your opponents face turn from elation (after rolling 8 crits and a Stun!) to surprise and defeat!
Even an attacker might use Divine Intervention after making an attack, just before the Counterstrike Step, to avoid a nasty Counterstrike.
Using spells in such a manner adds some great tactics and surprises to the game.
However, you can't change prior events. For example, revealing Divine Intervention after the Apply Damage and effects Step might whisk you away, but you still took the damage and effects.
Hope that helps!