ELI5 on ABC's checkpoint:
A line of code which looks like:
{556767, uint256S("0000000000000000004626ff6e3b936941d341c5932e"
"ce4357eeccac44e6d56c")}
This basically means that the hash of block 556767 should be the value in the quotes above. Hashes can't be generated on demand (and are not deterministic).
Shadow mining, if done by SV after block 556757 is useless and needs to be redone since at least one block before that. Therefore, it helps prevent re-org attacks.
