A Shin-rub is an unconventional, yet affectionate good-bye iteration of a secret handshake done by rubbing the shins and smiling. It will be awkward for everyone watching, but it doesn't matter because all the cool kids are doing it. It is customary to say "I'll miss you" to the other person while rubbing the shins, but it's not required.
You are about to leave a pool party, so right before you leave, you and your cool friend (who is in on the secret shin-rub) stand a few feet apart from each other to attract attention. In synchronicity, bend down and rub your shins vigorously, then proceed to say, "I'll miss you!" Hug, or do whatever feels right after. By now, everyone else should be completely awkwarded out. Look around and laugh at everyone with your partner, and leave.