Canon EOS Rebel T6i EF-S 18-55mm IS STM Lens Kit Instruction Manual - Page 211

Using AF to Focus (AF Method) Notes for AF AF Operation  Even when focus is achieved, pressing the shutter button halfway will focus again.  You cannot focus on a moving subject continuously.  The image brightness may change during and after the AF operation.  Depending on the subject or shooting conditions, etc., focusing may take longer or the continuous shooting speed may become slower.  If the light source changes while the Live View image is displayed, the screen may flicker and focusing may be difficult. If this happens, exit Live View shooting and execute AF under the actual light source.  If [u+Tracking] is set, magnified view is not possible.  When [FlexiZone - Multi] is set and you press the button (or tap on the screen), the center of the selected zone (or image center with automatic selection) will be magnified. If you press the shutter button halfway, the display will return to normal and the camera will focus.  When [FlexiZone - Single] is set and you press the button (or tap on the screen), the area covered by the AF point will be magnified. Press the shutter button halfway to focus in the magnified view. This is effective when the camera is attached to a tripod and you need to attain very precise focus. If focusing is difficult in magnified view, return to the normal display and use AF. Note that the AF speed may differ between the normal and magnified views.  If you magnify the view after focusing with [FlexiZone - Multi] or [FlexiZone - Single] in the normal view, precise focus may not be achieved. When in magnified view, Continuous AF (p.204) will not be executed. 211