Turbo mode switches on if your internet connection is slow (Turbo mode is based on Opera Turbo technology, developed by Opera Software). It helps speed up site loading and saves on data traffic. Pages load faster due to reduced data volume (page contents are compressed on Yandex servers before being sent to the browser).
Two thresholds determine when Turbo mode switches on and off:
If the connection speed exceeds 512 Kbit/s, Turbo automatically switches off and the icon disappears. If the connection speed once again falls below 512 Kbit/s, Turbo mode will remain off until it falls below 128 Kbit/s.