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The service is voluntary (each partner has to agree to share their present and future locations with each individual partner). The tracked person can turn tracking off and on at any time.The location is sampled about every 15 minutes, and on the map you see a spot saying "person Y, 3 minutes ago". Only the most recent location is shown: you don't see where the person has been in the past, although of course you could keep looking every 15 minutes. If the phone is turned off or the GPS is underground, the tag can say "person Y, 5 hours ago" at your last known location.Google keeps a person's location history in their Google account, accessible only to the tracked person and nobody else. You can see where you've been in the past week on the map, a few stats like how much time you spend at work in a given week, etc. The history can be deleted manually.It works with Android and iPhone smartphones. To be effective, it requires some data transfer over the cellular network, maybe a few megabytes a month.Google also uses the data to profile you for advertising and other purposes.