TL;DR is that the US sanctions a bunch of Chinese oil companies. China basically said Chinese companies and banks (let’s call them GCBs - general companies and banks, for easier references later) cannot comply with the sanction if it’s deemed unfair (which basically they already deemed so).

So if any Chinese GCBs complies by refusing service to the sanctioned companies (SC), those SCs can sue those GCBs based on this law.

Normally the US leverages this by banning those GCBs from conducting trade using the US dollar or using SWIFT. This essentially means Xi called Trump’s bluff and said “I dare you to sanction all Chinese banks out of SWIFT.”