To summarize (again), this is where someone whose household income is too high for them to qualify for ACA tax credits (400% of the Federal Poverty Line) chooses an ACA-compliant off-exchange Silver plan instead, which is either identical or nearly identical to the same on-exchange policy in every way except that the additional CSR load hasn't been tacked onto it.
Here's a perfect example found by Louise Norris...ironically, this is via Priority Health here in Michigan, which (until today) I thought was a "Silver Load" state, not "Silver Switcharoo". I'll have to do some more research to be sure, but it sounds like at least one MI carrier (Priority) is going full Switch:
NY State of Health Enrollment Tops 4 Million
Open Enrollment for Qualified Health Plans to Begin November 1, 2017
ALBANY, N.Y. (October 5, 2017) - NY State of Health, the state's official health plan Marketplace, today announced that more than 4 million people have signed up for health insurance through the Marketplace. Enrollment in the Essential Plan continues to grow, with more than 680,000 enrolled. The Marketplace is ready for its 5th Open Enrollment Period, which begins November 1, 2017 and has been extended to January 31, 2018, beyond the federal deadline of December 15, 2017, to allow New Yorkers sufficient time to enroll.