4. The Earth Lift
Most modern DI boxes have an 'earth lift' switch. This removes the earth from one end of the balanced line, and can be used to solve 'ground loop' problems, which typically manifest themselves as a nasty buzz or hum. Ground loops are very common when there's a lot of other audio gear and/or lighting in use. However, it's important to remember that the earth lift cannot be used if phantom power is required (see below).