This is only my observations and experience.
Magpies only swoop on the short period of time when their young leave the nest but cannot fly.
If you get swooped it is probably because you are unwittingly very close to the helpless chick.
First bit of advice is wear a hat.
If someone tells you they always get swooped then my advice would be now is the time of the year to locate the nest and try to avoid it. People say waive your hat or a stick but as magpies are extremely intelligent all as you are doing is introducing yourself as an enemy and they will always remember you.
I was hit very painfully by the family in the next territory so I now occasionally give them a tiny bit of food and it hasn’t happened since in fact I have picked up one of their chicks and put it back in a tree.
I think another trigger for attack is the speed of your approach. It is just my theory but a postie or push bike is a similar speed to a predator bird such as a hawk or eagle and this triggers an automatic response.
I read a book once which I certainly would not recommend where the author describes being called to a primary school and was met by a group of frantic parents sending their screaming children off one at a time to the school gates into the arms of the waiting 1st aid team. I am sure if a large group of parents and children calmly and slowly walked across the road then there would have been much less blood shed.
That short period when the chicks can’t fly is interesting. A few years ago we had a chick that could do no more than flutter but each morning it was to be found safely up in a tree. A friend told me she had witnessed the answer. She saw a chick being helicoptered by and adult which had picked it up by the back and was lifting it to safety.
Another reason for the blog is I was sick of overhearing comments like “my son saved a baby magpie yesterday”
followed by “what happened to it?” “Oh the cat got it”
Again just my opinion the best thing to do is just make sure it is safe and stay well back and see if the parents come back.
Put it in a safe place as high as possible.
Only if you are sure it has been abandoned (eg 2 dead adults) should you rescue it and contact the nearest Wildlife Rescue or sympathetic vet for advice. If you feel you have to move it note exactly where it was found as it will be accepted back by the family quite a while after separation.
Sorry about the rant.