I've set out previously how I think such a mechanism could work, building on the commitment in the December 2017 Joint Report
For some reason & to DUP annoyance, this dropped out of the final Withdrawal Agreement.
It should be re-introduced.
The purpose would be to create a mechanism to question new EU acts affecting NI only – it would specifically address the flow of new rules, not the stock.
It would only apply if the UK was in the backstop, and if the Assembly was restored.
You could require a strong majority vote to trigger the lock (although that could entail reforms to the petition of concern process).
✖️ One would be just to not apply the new act in Northern Ireland (nor, of course, in the rest of the United Kingdom) while working to resolve any issues this created.
✖️Another option would be, with the consent of the UK Parliament, to apply the act across the whole UK (meaning Westminster voting to keep the UK in step with the EU).
✖️A third option could be for the the Irish Republic to decide to seek a derogation from the EU for that new act. There are already existing EU precedents for this – for example, Malta has provisions limiting the purchase of second homes.
Because a) they want the deal to get overline and b) because it changes HOW not WHETHER the backstop operates
What the lock would do is give Stormont a veto role as part of the existing UK veto in the Joint Committee