Private keys in keystore. Which one is used while handshaking?

Private keys in keystore. Which one is used while handshaking?

I am creating adapter for work with web-service, using proxy and ssl
connection, and Have one problem with understanding Handshaking mechanism.
I will explain one example. My actions: 1) make server. Create private key
with keytool. create .cer file for it. 2) make client. Create same for
him. 3) exchange .cer with each other, import in keystores other side's
certificate. 4) create connection. Ok. Everything is ok. 5) in server's
keystore I gen new private key.
q: which pr_key do server use for handshaking?