Oval 3, if an approach is made in icing conditions, it is assumed that the aerodynamic surfaces are contaminated by a degree of ice. This remains the case unless it is visually confirmed that such contamination does not exist.
To give a margin of protection against the degraded performance of these contaminated surfaces, the incresed ref switch is activated. This increases the speed at which the aircraft stall protection system operates - ie the stick shaker and stick pusher. If this switch is used for approach and landing, the pilots must increase the speeds they fly at so as to maintain the same margin above stall protection system activation.
For a flap 15 landing, this increment is 20 knots.
The Embraer 145 has a similar system. It activates automatically with an ice encounter. Even if you subsequently fly an approach in non icing conditions in a hot climate a thousand miles from where you had your ice encounter, it cannot be reset, and you need to increase your landing speeds.