Technique:
When you brush the card, (if the card is spinning clockwise) move your hand diagonally up and left(but mostly with a quick upwards hit). Hit it with an open hand, fingers apart a bit, and relaxed. Let the card hit your hand as if it is a juggling ball you are about to catch. Except instead of catching it, hit it with a quick bump so that it continues spinning, and flies back up in the air. You will get the best brush if two corners hit your hand instead of one. You'll know when this happens because you'll feel the card edge hit your hand in two spots.
Aiming:
You can aim where the card is going to fly by changing the angle that your hand is, and the angle that you move your hand. Experiment with different angles when brushing the cards. You'll notice that if you have your hand parallel to the ground when you brush it, it will fly to the right because it is spinning clockwise. Angle your hand a little bit left to keep the card moving as directly upwards as possible.
You'll also notice that the angle that you move your hand in affects where the card flies. Ultimately you want to move your hand diagonally upwards and left. Looking at the unit circle, you want to move your hand between 90 and 180 degrees. The closer it is to 90-the more right it will go. The closer it is to 180-the more left it will go. In other words, if you have more upwards motion than left, the card(spinning clockwise) will bounce more to the right. If you have more leftward motion than up, the card will bounce more to the left. However if you hit the card with a hand moving 100% left, your momentum will just add to the spin of the card, and it will continue its downward motion. And if you move your hand 100% upwards, then your momentum will only contribute to the card bouncing back to the air, but not the spin - resulting in a card that will just tumble through the air. So you want to find the right amount of leftward and upward motion, but never 100% of either.
Aiming is better learned by trial an error, but I hope this helps in the theory behind the aiming of brushing.
Practice:
Start by holding the card in your right hand, throw it up, brush it with your right hand, then catch it.
Then try throwing it up with your right hand again, and brush it with your left hand, then catch it.
When you have done that enough times that you are pretty comfortable in controlling where the card goes, try brushing it twice with the right hand and catching it, and then twice with the left hand and catching it.
Then try brushing it with the left then the right, or the right then the left.
See how many times you can brush before you lose control of the card
Try brushing with different parts of your body(feet, forearms, biceps, legs)
Incorporate brushing with other moves(see Max Vlassenko for inspiration)
Tip:
Use cards from a new deck. A new card that is firm and straight will brush with way more control than an old card that has more moisture in it, and is bent from moves like springs or one handed shuffles.
Practice! I hope this helps!
-Matt Lang,
LangFlourishing.com