This was the PMQs defeat Jeremy Corbyn had been threatening to deliver to the prime minister since he became Labour leader. He found the right question to ask David Cameron, ignored his evasive tactics and hammered him with it, Paxman-style, over and over again. He took a subject which damaged his opponent and used the opportunities offered by PMQs to make it much worse for him. It was sturdy, convincing stuff.

The prime minister is not, at bottom, a very good debater. This is partly why he was so desperate to escape leaders' debates at the election. He really only has three tactics in response to difficult queries.

Usually, he answers a question on process with a statement on goals. If you ask how he is going to protect those who are having their tax credits cut, he answers by saying we need a high-pay, low-welfare economy. It is irrelevant, but it sounds like the kind of thing almost everyone would agree with.

Alternatively, he lists related government policies. You ask how he is going to protect those who are having their tax credits cut and he answers by citing changes to the income tax benchmark or rising employment figures. It is broadly relevant, but it does not answer the question.

Finally, when he's desperate, he falls back on the standard 'Labour will wreck the economy' line. So if you ask how he is going to protect those who are having tax credits cut, he just says you're a "deficit denier". It is irrelevant, childish and logically pernicious.

He tried all of those tactics today. Corbyn began the session by noting the Lords' vote against tax credits and asking the kind of question designed to maximise the prime minister's discomfort: would he guarantee that no-one would be worse off next year as result of tax credit cuts? Cameron was all over the place. He tried all three of his debate tactics, but Corbyn commendably stuck to the question. He asked it over and over again.

He has improved really rather quickly, from the man asking one-off questions on every topic under the sun in his first session, to being more focused but still letting the PM off the hook in the second session. In this – only his third – he had a laser-like focus. He didn't just stay on tax credits, he stayed on the same question.

Ed Miliband always used to give up. He'd ask the question once, twice, maybe a third time in the most extreme cases, then conclude he wasn't getting anywhere and try a new tactic. It was as if he retained an essential faith in Cameron's decency, a belief that if you asked the right question in the right way perhaps he would finally answer. He never did. Moving on only served to take the spotlight off his attempt to escape scrutiny.

Corbyn stayed on point, relentlessly. "This is the time we ask questions to the prime minister on behalf of the country," he reminded the Tory leader. Occasionally he would step back and make the broader case against Cameron – the fact he strongly suggested tax credits wouldn't be cut during the election campaign, the way those sympathetic to his politics have turned against him, the hostility of usually sympathetic newspapers – but then he rallied back to that one single question again, the one he knew Cameron couldn't answer: would he guarantee no-one would be worse off next year as result of tax credit cuts?

Cameron veered wildly. He tried some pre-prepared attack lines ("an alliance of the unelected and unelectable") and Tory MPs jumped on them rather desperately. The volume of their support gave some indication of how rattled they were. But none of it worked. Corbyn stayed on point.

For his final question Corbyn seemed to retreat for a moment, citing an email from a member of the public called Karen. The Tory benches started mocking him. "Aha," various Tory MPs said loudly, with the usual theatrics. But Karen's question did not get Cameron off the hook. Why was the prime minister hurting working families, she had asked? Corbyn read it out, gambling that perhaps Cameron would feel the need to answer her even if he would not answer him. And then he asked the question again, for a sixth and final time. And again, predictably, Cameron dodged it. But it was clear to everyone in the chamber, whether they were MPs, journalists or members of the public, who was on the back foot.

Cameron was calm and seemed largely unruffled by the experience. And it is worth noting that no PMQs performance is going to suddenly turn Corbyn into an election-winning machine. As is frequently pointed out, William Hague wiped the floor with Tony Blair almost every week at PMQs to no discernible electoral effect.

But this was still a hands-down, 6-0 victory for the Labour leader. He has found a way of throwing the harshest possible light on one of the prime minister's most disreputable habits: his refusal to ever answer a question. It is a potentially toxic weakness for Cameron and one Corbyn has quickly learned to capitalise from.