In Chapter 10, what does the woman do to Katniss? What the purpose of it?

In Chapter Ten, the woman inserts a tracker into Katniss' with a syringe. This metal tracking device allows the Gamemakers to know where she is at all times.

I expect the ladder to release me then, but I'm still stuck when a woman in a white coat approaches me carrying a syringe. "This is just your tracker, Katniss. The stiller you are, the more efficiently I can place it," she says.