You can only get pregnant on up to 7 days in a cycle: These 7 days include both ovulation itself, when the ovum can be fertilised for about 12-18 hours, and the days before, because sperm can survive in the body for up to 5 days.
trackle reliably detects these days (as well as the infertile days) according to the rules of the symptothermal method and marks them in your app. The STM has been researched in several studies and is very reliable - it has a Pearl Index of 0.4 for method reliability. With the symptothermal method, the core body temperature and the cervical mucus are evaluated. This provides a "double check" so that fertile and infertile days can be distinguished from each other very precisely. :-)
If you would like to learn more about the rules of the symptothermal method, please have a look here.