The number after the slash is the time increment. Every time a player makes a move, that many seconds get added back onto his clock. In your case the time increment is 10 seconds, so if a player moves faster than 10 seconds, he's actually increasing the amount of time left on his clock with every move.
If you don't want that, choose either a time increment of zero, or at least a smaller value.
How come while playing 15/10 chess live a players time will increase on me? Or in one instance it went to over 16 minutes after 12 minutes of play? seems very unfair.