> have never been sure how summing together something that is supposed to have no relationship with time magically provides an estimate of anything

Most people dramatically underestimate the amount of time something requires. As long as you give them a clear conversion rate between story points and hours, they will estimate the task in hours -- incorrectly, despite having made the same mistake hundred times in the past -- then convert the hours to story points and tell you the result.

Then someone notices that you have like 200 man-hours in sprint, and you have only selected story points for 100 man-hours. Which in fact is perfectly okay, if you understand that the "100" is the underestimate, and the realistic estimate would actually be close to 200, so you should be happy about the plan! But most people will not get it, and they will insist to plan properly for 200 man-hours. If you don't have enough political power to stop them, they will make you plan for 200 man-hours.

Then at the end of the sprint, everyone is stressed out, and they only completed 50% of planned stories. Because they underestimated how much time the tasks would take... just like research shows humans always do, no matter how many times they got burned in the past, no matter how much you yell at them to make better estimates.

(By the way, the problem with making realistic estimates is not just that individuals suck at it, but also that social forces actively prevent it. Research shows that people who make more realistic estimates are considered less competent than their colleagues, precisely because everyone notices that their estimates are longer that they believe they should be. And no one later changes their opinion just because the estimate turned out to be correct. Like, really, people who estimated something to take 2 weeks and delivered it in 3 weeks were judged as more competent by managers than people who estimated it to take 3 weeks and delivered in 3 weeks. The former made a better impression at the beginning, and the latter didn't provide a better result at the end, so the former made a better overall impression. This is how human brains work.)

So the smart way out is to make a metric that is taboo to convert to hours. Give vague verbal descriptions, like 1 is "trivial", 2 is "fairly easy", 3 is "simple", 5 is "medium", 8 is "kinda difficult", 13 is "tricky", and 21 is "needs to be split to smaller stories". People will first feel weird about it, but then they get used to it, and they will start delivering consistent ratings... like, the kind of story that gets assigned 5 story points in January will probably also get assigned 5 story points in December.

Then all you need is calculate velocity, which is, well, the conversion rate between the story points and hours. But you can't say that, or it will ruin the magic! You just say "during the last sprint, we implemented 50 story points, so for this sprint we will also plan 50 story points", and hope that people will accept that, without making the conversion explicit. And it works...

...until someone says: "Hey wait, so if we have 200 man-hours and plan 50 story points, that actually means that 1 story point equals 4 hours, right? And why are we giving this specific story 3 story points? 12 hours sound too much to me, I am pretty sure we could do it in 8 hours, or even 4 hours if we work hard, right?" (The rest of the team is silent, either because they agree, or they don't want to be seen as less competent.) And then you get another sprint when people plan too much, complete 50% of it, and get another stern talk about being more careful about making estimates.

It is a psychological trick that only works if you stop estimating stories in hours. It always breaks when someone insists on connecting the dots, converting the estimate to hours, and "fixing" it because it is "too much". If we could reliably estimate stories in hours, we wouldn't need story points, but experience shows we can't!

(But if you tell this to people, they will insist that they absolutely can make proper estimates, or that professional developers should be able to make proper estimates. Well, they can't, and we don't live in the should-universe.)

> I live in the real world so I estimate in hours

Do you make your estimates in front of other people who sometimes second-guess them? How often you actually meet your estimates?