HidrateSpark PRO (formerly STEEL) can blink after you take a drink from the bottle, showing that it has processed the drink you just took. The Sip Glows switch is on the Settings -> Reminders -> Bottle Glows screen (or the Bottles -> Bottle Glow Reminders screen, is the same) and is marked "Glow when I take a drink HidrateSpark PRO (formerly STEEL) only.
Please note, when turning Sip Glows on/off, the bottle has to be connected to your phone.