Complex sentence: In addition to Upgrade headers, the client sends a Sec-WebSocket-Key header containing base64-encoded random bytes, and the server replies with a hash of the key in the Sec-WebSocket-Accept header.
Split into simpler sentences: In addition to Upgrade headers, the client sends a Sec-WebSocket-Key header containing base64 encoded random bytes. The server replies with a hash of the key in the Sec-WebSocket-Accept header.