If you are US person and are a significant shareholder in a Controlled Foreign Corporation, then the US wants to tax you on your accumulated earnings and profits at the end of 2017. The timing of this tax was so swift that
you probably couldn't have planned for it. This is how it impacts you in a very general way. The tax considers your Earnings and Profits (similar to Retained Earnings) at the end of 2017 and then calculates a tax on your net assets at 12/31/2017 or an average of 12/31/2015 & 12/31/2016 for Cash Equivalents.
I think the thinking is that hey you have been holding your earnings in an overseas corporation and not exiting the money and therefore not paying tax, so we would like to treat this income that has not been distributed as effectively taxed. This creates a tax bite which is required to be paid April 2018 and the IRS does allow for installments over 8 years interest free.
The forms aren't ready for the tax, this income will be subpart F income and really the currently 1040 forms do not cater to the calculation. All of these clients/taxpayers will have to go on extension pay a reasonable estimate of the tax owed and wait for the carpet to be woven.