Well according to the info in the other thread the main difference appears to be in the base sauce which is high in aromatics, particularly cassia. Now that's all well and good if you make the curry with just the base, so the reduced base is the curry sauce, but the minute you add other spices it basically becomes a normal BIR curry. So, on the assumption that all proper (Birmingham) balti curries are distinguishable from a standard BIR curry, and I've no idea whether that assumption is valid or not, I like you am at a loss to understand how that is achieved.
At a guess maybe the balti mix powder that is used for balti curries is heavy on the garam masala to retain that USP of an aromatic curry. And of course it's cooked in a karahi and they claim it's the constant catching fire of the oil that lends an extra dimension to the dish. Anyway what most BIRs present as balti is anything but, being just a standard curry served in a karahi dish. I have bad memories of this as my last curry at a BIR was a "balti" and god was it bland. Or is that deliberate, is that what makes it a balti? It's annoying not to have had any experience of the real deal.