It's called a "loading dose" and it's a common trick in medication dosing.
If your drug has an elimination half-life of 1 day, then it takes about 4 days to reach the steady-state concentration if you take normal doses. With the loading dose, you immediately reach it.
The evidence I’m aware of suggests that this method (on-demand or event-based PrEP) is broadly as effective as daily PrEP (though it’s not suitable for vaginal sex). I believe adherence rates are generally lower though, as people can find the instructions confusing, and it’s obviously easier to mis-dose than a daily regimen.
If your drug has an elimination half-life of 1 day, then it takes about 4 days to reach the steady-state concentration if you take normal doses. With the loading dose, you immediately reach it.