What is the purpose of indirect checkpoint when the default checkpoint time (when recovery time is 0) does a 1 minute checkpoint?

Indirect checkpoint is performed when the target recovery time is configured. When the value is 0, then it will do checkpoints such that default recovery time is 1 minute. Where as if the value is greater than 0 (recommended value is 60), then SQL server will do indirect checkpoints. I'm trying to understand menaing of indirect checkpoint with an example - Suppose value of 40 is configured, then does it mean that: - SQL server does checkpoint so that recovery time is 40 seconds? Then what is the difference between automatic and indirect checkpoint?