Card trick question

Hello,
A few days ago a friend showed me an awsome card trick but he wont teach me.
Ill try todescribe the trick the best i can and hopefully you guys can help me.
He took the pack, counted 10 cards and gave them to me.
He told me to count the cards myself and so i did. I had exactlly 10 cards.
Then he told me to put the cards on the table and cover them with my arm.

Then he said some ?magic" words and told me to take my arm off the cards and count them again. Well guess what, i had 13 cards!

He then told me to cover the cards again, take off my arms and count. I had 16 cards!
How did he do that?!

Thank you for your help, Alex.