It is based on this tweet:
With extra detail here:
Let's call it "infinite transition to avoid the Backstop"
The best they came up with was to replace it with "alternative arrangements" but could not articulate what those were.
Instead of the Backstop being needed when Britain's EU transition period ends at end of 2020/22, the transition wouldn't be time limited.
Strikes me they'd have to be highly foolish to go for this. But then again... maybe they are?