Believe it or not, I can do that, inlcuding behind the back throws. I'm actually a pretty good juggler.
All she's doing is one hand juggling, but with both hands at the same time. To be juggling you must have more things to juggle than you do hands juggling them.
In her case, she has two balls for one hand, this is the most common way to juggle 4 balls. Notice the same two balls are always juggled from the same hand.
To juggle 3 or 5 balls, she would be crossing over where each hand gets a touch of each ball.
You can learn this too. Pick up two balls or lemons in one hand, throw one up in the air and right before you catch it throw the other ball in the air. Only involve the original hand.
Notice that to add a third ball you simply throw the ball to the other hand and right before you catch it, you throw ball your holding back to the other hand. Notice, in this method of juggling, there is only one ball in the air at a time.
The other method of juggling 3 balls is to throw them in a circle, like you see in cartoons. In this method, there are 2 balls in the air at the same time while one is transferred between hands...
I used to juggle 3 lemons and catch one on the back of my neck, straighten up, and let it roll down my back and I could kick it with heel back up over my head to keep juggling.
That's actually easier to execute than juggling 5 balls. Which I can do, but not nearly as well.
Been a while since I've juggled in earnest, but it's a lot like riding a bike. I can pick up any three things and juggle them.
I can even juggle scarves, which could be the hardest things of all to juggle, even if it doesn't seem like it...