Hi I have a question about Batman Begins

Hi I'm ShehbazAhmed I love DC and Marvel and I like to contribute here. I have a question about a specific scene in Batman Begins. The scene where Batman is in the Arkham Asylum building where Rachel is poisoned. The part where Batman summons the bats to come into the building to distract the S.W.A.T. in the building while gordon takes rachel out and then batman throws this thing from his shoe which makes the bats follow him. What was that all about how did batman get the bats to come there what where they even doing?