[Ref Youtube, Boeing]
SLG (Semi Levered Gear) is a design that can be found in B777-300ER. A “diagonal” hydraulic strut provides the energy to provide better takeoff performance. The design is used to increase takeoff capability, or provide increased tail clearance for the same weight and thrust as nonequipped airplanes.
Below are two photos of triple-bogie. However, take a look closer you might find that the photo on the left is SLG found in B777-300ER and the other one is just the typical triple-bogie on B777-300.
So, why B777-300ER needs the SLG?
Because the vast majority of the weight of the airplane is borne by the lift of the wings at the time of rotation, the semi-levered gear (SLG) acts as if it were “pushing” down like a longer gear. This allows a higher pitch attitude for the same tail clearance OR more tail clearance for the same pitch attitude.
I don’t understand still – why not listen what Captain Joe says: