Chris Williamson, the Corbynite MP for Derby North is under threat of deselection in his constituency, Paul Waugh reports in the Huffington Post. The cause? Irritation among the trades unions at his vocal criticisms of the unique powers they have over the party’s selection processes and rulebook.

The story raises the prospect of one of the biggest advocates for full selections for sitting Labour MPs having to go through one, which is causing amusement among some Labour MPs. However, but the story is also an illustration of the very important change to how Labour’s re-adoption processes have changed, to the disadvantage of the trade unions.

Under the rules of Labour’s trigger ballot system, a sitting Labour MP must get the votes of 70 per cent of their local party branches and 70 per cent of their affiliate branches otherwise they must go through a full selection process in which they are merely one of a number of candidates.

But here’s the rub: under the terms of Labour’s selection process, there is an absolute cap on the number of local party branches – one branch for each ward. Derby North has seven: Abbey, Chaddesden, Darley, Derwent, Littleover, Mackworth, and Mickleover. The votes of these branches count equally, regardless of how many members are in each branch. So if just two branches do not vote to readopt Williamson as the candidate, he faces a full selection.

The rules get still more tricky as far as affiliate branches are concerned: party affiliates – like the trades unions, Fabian Society, LGBT Labour and so on – can affiliate an unlimited number of branches. They don’t even need to have members and unlike local party branches they have no requirement to ballot their members.

Of course, this essentially means that any trade union which wants to trigger a full ballot can do so, as it can simply affiliate as many branches as it needs in order to exceed the 30 per cent threshold. So in a way, of course, the trade unions have a lot of power.

Except in a full selection it is party members who are sovereign. They pick the candidate (the sitting MP is on the ballot by right so the trade union role over shortlisting doesn’t come into play) so as long as an MP remains popular with their local party, the trade unions can trigger them until the cows come home – it won’t matter. Williamson would only be in jeopardy if he also had difficulty among ordinary members (which according to the HuffPo he may do).

The big power the trade unions used to have under the old trigger ballot – when the votes of affiliate branches and local party branches counted together – was not to trigger contests but to prevent them. That gave MPs an incentive to be loyal to the interests of trade unions not the preoccupations of party members. (A good example here is Heathrow expansion: ultimately the pro-expansion trade unions can no longer protect MPs from being deselected by anti-expansion activists, a major game changer as far as the politics of airport expansion go.)

So regardless of the fate of Williamson, for the moment, Labour’s rulebook has shifted, not decisively but significantly, in favour of lay party members.