Spindle locks lock the chuck so one can change the bits safely. When the lock is engaged the chuck screwdriver or drill bit cannot move.
Most modern drills automatically lock the chuck when the variable speed trigger is released.
Some models have a manual spindle lock. The lock is the middle position of the forward/reverse switch.
When should you use the spindle lock?
Use the Spindle Lock when Inserting or Removing Drill Bits
The spindle lock is usually used when inserting or removing drill bits to prevent injury to the user.
Use the Spindle Lock when Manually Loosening or Tightening Screws
A spindle lock can also be used to use the cordless drill like a screwdriver. With the spindle locked firmly in place, the user can use the drill like a manual screwdriver.