The lh surge is what triggers the egg to release.
For some people the surge is gradual- so every day they'll have a bit more lh- the tests will get a bit darker each day Til it's positive. This puts you at an advantage because you get a bigger warning when you're going to ovulate and can get more bd in your fertile period.
Or, like me- Theres no warning at all- the test can be really faint one day, then positive the next, so no phase- in for me, but still helpful as you still know ovulation is going to take place within 48 hours of the positive test.
Hope I explained it ok! Xx