The Detective archetype for the Bard has a True Confession ability. I do not understand one part of it:
Using this ability requires a successful Sense Motive check to see through a Bluff or notice mental compulsion.
How does that interact with the rest of the ability, namely:
After 3 continuous rounds of performance, the target must make a Will save (DC 10 + 1/2 the bard's level + the bard's Cha modifier). (...) On a failed save, a liar inadvertently reveals the lie and the truth behind it. A creature under a charm or compulsion reveals the nature of its enchantment and who placed it
As I read it, it means to use this against a Bluff or to detect mental compulsion (which sounds like all the cases you would use it for), you need to first succeed at a Sense Motive check against the usual DC and then, after 3 rounds, the victim gets a Will save and on a failure they reveal something.
Wouldn't the initial Sense Motive check reveal everything anyway if successful? If so, what's the purpose of this ability?