How do I single display when I am screen Mirroring

[Android Central Question]

Hello, please may I ask for some technical help.

I have a Samsung tablet, and I like to screen movies from my tablet to my TV. However, when I screen Mirror from my device to my Samsung TV there is lag on the sound and its very irritating. By accident, the other day when I was screen mirroring my Samsung tablet, I think I pressed something and the screen was only projecting onto my TV and not on my tablet, this resolved the issue of the lag as it was only being screened on the tv and my tablet screen was blank although still streaming the movie on to my TV.
I have tried to do this again but I have failed and I can not work out how I did this, can someone help me please so I can watch movies on my screen without the lag. I think its called one display screen but I have no idea.