Under Johnson’s proposal, NI would stay in the single market if the N.I. assembly voted in favour. In the absence of such a positive vote, NI would leave the single market.
NI Assembly rules mean that on key areas (known as Petitions of Concern) a simple majority of members voting in favour is not enough: there needs to be a majority of Unionists and a majority of Nationalists.
As the default is that NI leaves the single market unless there is a vote in favour, this means that in order for NI to stay in the Single Market, the DUP would need to vote in favour. Chances of this happening = zero.
This DUP veto is against the wishes of the majority. A majority of NI assembly members have written to Donald Tusk to say they support the backstop. Similarly, a majority of Members have voted for gay marriage but the DUP used its veto to block it.
Johnson’s proposal in effect is saying that NI can stay in the Single Market if the DUP agree.
There are alternative ways of gaining consent - e.g, a simple majority of NI Assembly members to approve staying in the single market or a N.I. referendum on the single market. Both of these are highly likely to be approved as the DUP is not representative of the majority of N.I. opinion.
Johnson’s ‘consent mechanism’ is a figleaf designed to sound reasonable to his supporters while conceding nothing in practice.