Calling the HMAC object with a password returns a new object that can perform the actual HMAC hashing. E.g. doing a HMAC hash with MD5 and the password "bar" of the string "foo" would require the code Crypto.HMAC(Crypto.MD5)("bar")("foo").
"bar"
"foo"
Crypto.HMAC(Crypto.MD5)("bar")("foo")