Jeremy Corbyn has kicked off his campaign for the Labour leadership with what is being seen as a coded threat to deselect his dissident MPs unless they fall into line. Jeremy Corbyn’s aides, meanwhile, have insisted that he is simply reiterating the current state of affairs.

The reality is, without rule changes, boundary changes are an imperfect tool with which to remake the parliamentary Labour party to be more Corbyn-friendly.

Why?

There are two angles to this which need unpicking – the first, which is boring but essential to understanding the Labour party, is how the party’s selection processes actually work, particularly around boundary changes.

“Deselection” is a term that is frequently bandied about when the Labour party is discussed but poorly understood. What no longer exists is “mandatory reselection”, when an MP has to go through a full selection process. MPs can still be deselected if they fail to secure a majority of votes in their trigger ballot.

That word “votes”, as so often in the Labour party, is somewhat misleading. Each constituent “branch” of a local party has one vote. The membership branches of a local Labour party get one vote each which is decided by a ballot with a simple “Yes/No” option on whether to keep the sitting MP or trigger a full selection. But so does the local branch of any affiliate organisation, be that the Fabian Society, BAME Labour or a trade union. The votes of trade union branches – and often, in practice, most affiliated societies – are decided by a local official, not by a full ballot of local members.

That means, in practice, the sitting MP can lose the support of their members and keep their position provided they can keep their local trade union officials on side, and vice versa. (My expectation, however, is we will see a number of MPs facing full selections in the coming years, regardless of who emerges as leader in September.)

A “full” selection process only comes about if an MP is unable to win their trigger ballot. (That’s why Corbyn’s use of the word “full” and his description of a process which the sitting MP is one of a number of candidates has spooked the rebels).

But what about when there are boundary changes?

In the case of boundary changes, the MP remains the sitting MP provided their new constituency comprises at least 40 per cent of the old.

This is where things can get tricky. Let’s say for example you start with four seats, all Labour-held, two safe and two marginal. (Let’s call this place “the Wirral”. It’s just a name.)

Let’s imagine you are a Corbyn sympathiser who wants to get rid of one of the MPs on the Wirral. (Let’s call her “Angela Eagle”. It’s just a name.)

The boundaries are redrawn – one seat vanishes, leaving two safe Labour seats – and a third safe Conservative seat. At this point the game of musical chairs starts, with one person certain to end up without a seat and a fourth set to end up de facto unemployed.

But here’s the problem – one of the four MPs has lost their claim, as there is no remaining seat which contains more than 39 per cent of their old seat. (Except perhaps the safe Conservative seat, but let’s assume they find this an unappetising prospect.) But all of the remaining three MPs have a 40 per cent claim on the two safe Labour seats (let’s call these two seats “Wallasey” and “Birkenhead”. They’re just names.)

So this presents you with a golden opportunity to get rid of your opponents, right? There’s just one fly in the ointment: the other sitting MPs are Frank Field and Margaret Greenwood, only one of whom (Greenwood) is a particular friend of Labour’s current leader. This pattern is fairly similar through the country – Corbyn’s opponents tend to be neighbours, and vice versa. (It’s perfectly possible, for instance, that boundary changes will create one Islington constituency, resulting in a face-off between Emily Thornberry and Corbyn himself.)

Of course, he could change the rules. However, by my reckoning, even if, as looks likely, all six of the positions elected by ordinary members on Labour’s NEC go to the candidates of the Grassroots Alliance, there still won’t be a majority on the ruling exec to bring about wholesale changes to how Labour selects its MPs.

What feels more likely is, as one senior ally of Corbyn’s reflected at the start of a coup is that Corbyn’s re-election campaign run on a “package to democratise the party further and put members back in control – the stuff MPs’ worst nightmares are made of”, before using that as leverage to bring back the bulk of the soft left and the “make it work” caucus on the right of the party into the tent.