How does this Grant Access Using Hierarchies work?

When Grant Access using Hierarchy is checked for custom objects(For standard objects this is always checked) even if the record is private ,users higher in the role than the owner of the record still get access to the records . A simple example can be to assume opportunity is private OWD and you are Sales Rep owning an opportunity record ,you have your manager who is higher in the role hierarchy than you ,even though opportunity is private he will still get access to your records . Technically a for every user who has access there will be a share record in the Share table of the Object.