The IRS withholds 30% of gross performance income from a foreign artist the moment a pays them. A Central Agreement can cut that number dramatically, but only if it is applied for well before the first show.
A UK act going to the United States faces a withholding regime designed for a worst case: that a foreign entertainer will take the money and leave, never filing a US return. The default rate is 30% of gross performance income, withheld by the promoter (as a withholding agent) and remitted to the IRS.
30% of gross (not net) is a significant cash drag. On a $500,000 gross tour, that is $150,000 the artist never sees during the run, held by the government until a US return is filed and a refund (if any) comes through.
A Central Withholding Agreement (CWA) is an arrangement with the IRS that replaces the default 30% gross withholding with a reduced rate calculated on the tour's expected net income and the applicable treaty position. If the CWA is granted, promoters withhold at the agreed rate, and the artist's on tour is materially better.
The catch is timing: the IRS requires the application well before the dates, and issuing a CWA after shows have already occurred is not possible.