S767,
If you look at the skid (ALF) at the bottom of the actuator, above the scuff pad and slightly to one side you will see a small cylindrical object sticking out about 1.5 cm long and a cm in diameter. (It is just to the right of a charging valve which has a cap on it rather like those on the wheels)
If the tail is struck this may allow the charging pressure in the strut to escape. The low pressure causes a red pin to extend from the cylinder showing servicing is required. (The pin will also extend if the pressure drops below the nominal precharge pressure of 250 - 300 psi due to a leak for example (actual figure for indication is 174 - 232 psi)
A more serious strike will cause a shear pin to break protecting the fuselage from possible damage.
Hope this helps
Cheers.