A differential lock temporarily locks differential action so paired wheels or axles turn together, improving traction when one side is slipping.
Why an open differential exists
On a normal turn, the inside and outside wheels travel different distances, so they need different rotational speeds. An open differential allows that difference. The downside is that a wheel with very little grip can become the limiting path for usable torque.
Front and rear lockers
A front locker links the left and right front wheels; a rear locker does the same at the rear axle. They are powerful off-road tools but can make steering or turning more difficult, so they are generally engaged only when needed.
What is a center differential lock?
A centre differential sits between front and rear axles in many permanent-4WD systems. Locking it forces the front and rear outputs to stay coupled rather than freely accommodating speed differences.
Grenadier Trialmaster example
INEOS Trialmaster models add front and rear differential locks on top of the standard centre differential lock. The switches are mounted in the distinctive overhead control panel shown in our original photo.