ACE inhibitors Nephrotoxic or Nephroprotective

I am really sorry but i still fail to understand the exact reasoning of ACEi cointraindication in Renal Artery Stenosis.

If ACEi increase the overall perfusion of kidney which is what is decreased in the Renal Artery Stenosis, why would it be contraindicated? I mean, wouldn't you want ATII to be decreased? This would lead to dilated efferent arteriole so even if its less blood coming from the afferent, at least we will send it more in the later part or something. I hope i am able to convey my confusion here and hopefully get a better explanation.

Mondoshawan said:

Because even though ACE inhibitors increase overall kidney perfusion, they decrease glomerular perfusion. That's great in something like diabetic nephropathy where glomerular hypertension is accelerating the glomerulosclerosis - but it could be dangerous in something like bilateral renal artery stenosis, where the atII-constricted efferent arteriole is the only thing maintaining the glomerular pressure that is needed to maintain renal function.

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None could have cleared my doubts any better. Thanks a lot dude!