I often find new features on the Canon C300 to help me work more efficiently. The magnify button is an essential tool, especially when shooting with a shallow depth of field. In default mode this has a small yellow sign notifying you that you are in magnification mode. I still find that I can forget I am in magnify mode for a second, particularly if I am using a zoom lens. There is a great feature that turns the magnified screen black and white. This is great for focus as the sharp section really pings into vision and it means you'll always know you are in magnification mode.
The feature itself is burried away in the menu, so there is a very brief video below to show where it is located. Essentially it is MENU>SETTINGS>CUSTOM FUNCTION>F.Assist. BandW>MAGNIFY