Excise Tax on High-Cost Coverage

Can anybody really explain this particular element of the ACA? My sister in law's company is cutting employee benefits to avoid paying the tax. In a nutshell she's getting a lower tier of health insurance and having to pay more out of pocket. I've read various explanations of the tax, but I've yet to clearly understand its benefit. I'm all for health care reform and I think that the ACA is a step in the right direction, but it looks like corporations are simply lowering the amount of health coverage they'll provide in order to avoid paying this tax. As I stated earlier, I don't have a clear understanding of this policy and I figured that this would be the best place to get an honest explanation. Thank you in advance!