Switch stick

I expect some hate lol . I don't know what it is. Back to 25 after a few years off. Can someone explain switch stick to me. I was looking up why my opponent only played y sail and it said to defend by utilizing switch stick

u switch the player that you are defending with by flicking your right stick into the direction to the player you want to lock onto 

the first player needs to be in a zone, if he is blitzing you can’t switch stick

And you can't be using a D-lineman.

Ty so much all. I was like man this is probably a stupid question