When the detect protocol function is enabled under an explicit proxy port (such as port 8080, 3128 or 80), the ProxySG can initiate a detect protocol mechanism and handoff the proxied connection to its appropriate protocol (HTTPS in this case).
In contrast, if the detect protocol function is disabled, the ProxySG will only treat the "tunneled" connections as a normal TCP connection, and "protocol-related" policy (HTTPS in this case) will not apply to it.