How can I effectively contest layups and dunks?

Whenever playing against my friend, I always find myself having a tough time contesting dunks and layups. For example, if I have a defender under the basket during a fastbreak, and the player on offense is driving for a dunk, my player just moves back and gets dunked on. But whenever I try this same exact scenario on my friend, his defender always interrupts the layup without it being blocked, and most of the time it usually goes without a foul being called.

To anyone who can effectively contest layups and dunks, what's the secret?

There's no real secret, the easiest way to kind block dunks and layups without fouling is to run at the player from an angle. It's pretty difficult swatting someone from in front without fouling them. It's mostly about timing, if you just stand under the hoop then the guy will dunk on him without doubt, if you're faced with a situation where you're under the hoop and the guy's coming towards you the best thing to try is to use the Turbo button to run right at the player then try to block him. If he doesn't block the shot at least he'll foul him. But yeah as I said, I find that running in from an angle is the best way to block dunks/layups.

If he doesn't block the shot at least he'll foul him.

Or in the best case scenario, force him to adjust his shot which isn't as good an attempt. Depending on the foul sliders, you might even get away with some contact which can often disrupt the shooter.

If you are right under the basket youve got no chance. Stand outside the semicircle the one under the basket, or your player may not be able to jump.
Somtimes it is best to allow some room between you and the driving player. If you do this with a decent shot blocker you will deny all but the best dunkers. Sometimes it is easier to block from behind even.

Best way I found in my opinion is to jump a bit early.

A tiny bit.

Its easiest when they are attempting a put back after a rebound.

But I can often get faked out. But is still worth it to see dunks and lay-ups miss or sent into the floor boy.