As I understand it, it's similar to all the other new riser slots, you put a cheap-o card in it that basically gives your chipset access to the outside world, so as it can provide you with LAN, modem or other devices without the need to buy a full-blown 'expensive' version of one of these.
Yes, I know what you're thinking right now
I have one of these risers on mine, different type (I think there are 4 standards for it, all massively unpopular) but I think mine will remain forever empty. I think I'd rather pay the extra £3 and save the CPU cycles
What does this button do?
Dave is exactly right, its another riser for software-driven add-ins like modems or sound. Very similar to AMR or CNR.
The problem with ACR is that it's a VIA proprietary standard. That is, it's VIAs, and you'll never see it on anything but VIA boards, where as Intel has made its CNR standard available to everyone (including VIA).
Because of that, I'm about 99.99% confident that ACR will go the way of Betamax. That is, nowhere. Those slots aren't popular as it is, and when they are used, people will be far more likely to use CNR (because it's an open standard) than ACR.
I know this is an old post, but I ran across it on Google.
ACR is NOT a VIA proprietary standard. It is managed by the ACR SIG, and while you have to be a member to put it in your chipsets, it sounds a lot better than anything Intel has. The standard was designed to be a better alternative to AMR and CNR. It allows cheap modem, audio, and ethernet cards just as Intel's standards do, but it also allows hardware acceleration and such to be added to the cards allowing a mid-level performance if not high.
You can actually see this slot on an SiS motherboard.
My mistake, I should not have said that it was 'proprietary' .. poor choice of wording. However, I still don't see ACR as going anywhere. Neither of the two are wildly popular in the first place (ACR or CNR), and even if it's slightly less 'potent', my money would be on CNR, simply because, like it or not, Intel is still the dominant force.
In any event, neither of those interfaces are of particular interest to the readership here, which is why we don't talk about them much.