What U-turn tells us about May's government

"Screech!" The tyre marks must be visible on the tarmac, the smell of burning rubber hanging in the air over Downing Street.

Almost exactly seven days later to the minute, the prime minister and chancellor have dumped the most controversial part of their Budget, scrapping the planned rises to National Insurance.

A U-turn so fast, so blatant, so complete, it's hard to think of recent examples that are so overt. Why?

Number 10 and Number 11 were worried about the fact they would have been breaking a manifesto promise to carry the measure through.

Their complicated excuse just didn't wash. Some Tory backbenchers were so unhappy about it that they had gone into overdrive trying to change the chancellor's mind. And in the end ditching the plan, while politically costly, doesn't cost that much money in the longer term, in the context of the whole government budget.

So what does it mean?

It undermines the credibility of the chancellor, an admission at the very least, that his political antennae have gone wonky.

Second, it questions the extent to which the prime minister is willing to back him.

Relations between the next door neighbours are businesslike and between their operations certainly frosty - and this will not have improved things at all.

It also tells us that although polling suggests the prime minister is strong in the country, she's simply not that strong in Parliament.

A firm nudge from backbenchers, and they shifted.

One Tory MP told me he was "livid… a little bit of difficulty and they give way?"

Given the complexities of what the government hopes to achieve in the next four years, for ministers to cave so quickly on this is worrying for some of their supporters.

However, whereas in traditional times, a reversal like this would have been a disaster for the Prime Minister, instead, by the end of today's Prime Minister's Questions Theresa May was leaning back and grinning, with the chancellor appearing relaxed alongside.

You could see from the faces of Labour backbenchers, Jeremy Corbyn was unable to land any blows.

Who would have thought it?

A giant, embarrassing reversal from the government, a gift for the opposition, but the Labour leader instead was the one looking uncomfortable by the end of his weekly clash with Theresa May. A bad day at the office for them both.