If you add about 1% salt to the hot water in advance, the egg white will not flow out when the shell cracks.
Salt slows down the coagulation of the protein, which prevents the egg white from leaking into the water through cracks.
It is said that you can use vinegar or starch instead of salt, but this is because vinegar speeds up the coagulation of proteins, and starch gives water stickiness and reduces resistance when the egg white tries to flow out. to prevent the egg white from spilling out.
There is also the question of salt and vinegar getting inside through the shell, but normal boiled eggs finish within 15 minutes at most, so there is no problem with the taste soaking in.