Mobile hotspot speeds are capped at 5 Mbps, with detected video streaming at a speed typically suited to 480p quality (around 2.1 Mbps) .
Depending on phone (typically iPhones) hotspot usage is limited to a single device at a time. Visible seeks to transmit video downloads or streams to smartphones at 480p for all plans - i.e. the throttle detected video streams. The key word there is "detected" - not all video streaming is detectable and in some cases other activity may be falsely detected as video streaming.
It's also described under the plan details at https://www.visible.com/plans - click on the Get all the details button for each plan.