The London bridge is a difficult, rider-on-top sex position that requires a great deal of stamina and strength on the penetrating partner's part.
It's similar to the basic bridge sex position, but requires the penetrating partner to make a bridge with their body, rather than the receiver. To get into this position, the giver makes a bridge with their body by laying on their back, planting their feet on the floor and raising their pelvis and torso above the floor with their hands. The receiver can then straddle them.