Well, this is a great question. And not a simple one either because using a bounce board is more complicated than it sounds. Many people think bounce boards are simple to use but they aren't. You have to understand the subtleties.
When someone first learns how to use a bounce board, they hold the board below the clients face and reflect light up all over them. They think this looks good when in reality, it actually looks terrible.
You never want light coming from below, unless it's extremely subtle. It's much better to have light coming in from the side.
This means that to use a bounce board correctly, it is best to use the dull white side of the reflector and shine it from the side of the actor's face instead of below. Rather than reflecting light onto the actors face, you are actually just adding some extra brightness to the side of their face.
But to really get a shot that pops, you can use two bounce boards on either side of the actor's face so that they shine on both sides and you have really great light coming in. It looks amazing and will get you excellent response to your shots.