Use a SSL cert to decrypt traffic

You'll need the SSL/TLS certificate AND private key installed on the BIG-IP. Then create an SSLClient profile and assign it to the relevant Virtual Server. You can then apply iRules and a host of other good LTM features. If you need to re-encrypt to the real servers too, you'll need to create an SSLServer profile and assign it to the VS too; this will contain the server certificate etc..