Why Q signals are three-letter shorthand from CW/traffic practice that carry a fixed meaning. QSY means "I am changing frequency" (or, as a question, "shall I change frequency?"). It is the one you hear when someone moves a contact off a calling frequency to a clear one.
Watch out QSL means "I acknowledge receipt" (and gives us the QSL card), QRZ asks "who is calling me?", and QRU means "I have nothing for you" or "do you have anything for me?"
QSY = "Shift Your frequency." QSL = confirm, QRZ = who's calling, QRU = nothing for you.