Here is a good explanation:  It is a very subtle article, so read carefully. Every country has a slightly different balance between executive, legislative and judicial branch, depending on the question where sovereignity actually lies and powers granted to each branch.  For example: Belgium, under King Leopold, had enacted a pro abortion law. The King felt he could not sign this into law, for reasons for religious and moral conviction. He stepped down. It did not mean, the law was not signed into law eventually, but it goes to show that every country has a slightly different way of dealing with the question where sovereignty resides and how it is expressed.  What I understand: A complicating matter is the position of Northern Ireland and Ireland. Theresa May negotiated an exit deal, that would effectively mean, the UK is leaving per a certain date in 2019, but would "remain" within the trading relations with the EU as if UK had not left at all till 2022. And during this transition period, the actual post exit relationship would then be ironed out. Of course, getting on without getting on is hardly a solution.  The basis for this complicating matter has to do with the Irish peace process. It was agreed that an open border should be in place. A no deal BRexit would yield the situation that only Ireland has an open border with the UK where goods, people and capital can flow across. At present, the level of "trade" is around 4 billion pounds. For that region, quite important, no doubt.  This presupposes that standards are the same. For instance, the EU prohibits certain GMO foods (US foods?). The UK leaves this standard, and allows it to be imported into the UK. This means that via northern Ireland and the Irish Republic, GMO foods wind up on the EU wide market, effectively rendering the prohibition void. And this would go for any tariff on US goods! On the other hand, it would also mean Irelands would become the trading hub for the EU to enter the UK market, without a hickup, and via tradedeals other markets like the US.  In other words, Ireland then would get a huge say in what goes on in the EU, upsetting the carefully designed balance between the two major continental powers: Germany and France. So, making the Irish backstop part of the leave deal sounds reasonable, right? No, because effectively, instead of leaving, it would mean remaining. And of course, the future relationship would remain unresolved, meaning that the UK has left, but is a subsidiary for all intends and purposes of the EU, without the sovereign power to hatch out any trade deals with third parties. So, it stands to reason, that crossborder goods can only go as far Northern Ireland or Ireland, needing a heft control mechanism. Taxation is a bitch, right?  What is not talked about is that it could reignite the Irish situation with all it's dire projected and unintended consequences. Especially, given the fact that Ireland also has seen an influx of aliens.  With the new act, that passed both houses in parliament,  the UK parliament has painted the executive into a corner. There is full confidence in the executive, no new elections possible because of the fixed term law of 2011. And by charging the executive to come up with a deal, while the EU is totally uninterested to let the UK exit from the Union, it was hoped, this would effectively put the outcome of the BRexit referendum: to leave the EU, on ice.  If a clean Brexit indeed would happen, and a nice trade deal is hatched between the UK and the EU, it follows, after instituting more tariffs on EU exports, the UK would be, instead of in an underdog position with regard to the EU, in a position of the top dog, as the UK effectively would become the gateway to the US for the EU (Read Germany, France, Italy)  Many BRemainers argue that it was never intended to be a no deal BRexit, as the referendum only speaks to leaving, not as to the how. So, in the interim, many MP's would love to see another referendum saying: leave or remain, or better said: not taking no for an answer, and many feel more and more deprived of their trust in democracy, whether they are Pro Brexit or pro-Bremain.    So, if Boris Johnson is as true to his word, as he claims to be, he has no other option than to advice the Queen to refuse assent. It certainly would make him a hero with many of the 52% exit voters, but also with a major percentage of the remain voters who feel that the will of the people should be implemented as promised.  From the US perspective, the whole thing is that Potus has a couple of buttons to push to make a few people feel pain. China is now pushed from several sides, and hardly winning, but rather seeing it losing more and more ground.