This aftermarket Trigger type switch with lock has 4 internal terminals and replaces Milwaukee 14-78-0550 switch. It is a 10A-125V Over Hang Trigger Switch that comes with Lock and integrated dust proofing commonly used in high rated bench top and power tools. 

Switches are commonly replaced parts in power tools. A switch handles all on/off and variable speed functions. They also have a role in electric breaking; they can momentarily reverse electrical polarity. Because switches use mechanical internals, they tend to degrade and wear out over time, eventually requiring replacement.

NOTE: Some common signs of a broken or failing switch are:

• Tool has completely stopped functioning - it will not turn on
• Tool operates intermittently, requiring miltiple tries to turn on, until eventually it won't turn on at all
• Variable speed function and electric braking aren't working properly