What is the difference between internal and external bypass?

BP-51: RACE TECHNOLOGY CLEVERLY PACKAGED FOR EVERYDAY 4×4 VEHICLES

Traditionally, the bypass shock design incorporates a number of tubes that run external of the shock absorber body in order for the hydraulic fluid to bypass the internal piston.

This design has prevented bypass shocks from being incorporated into everyday vehicles without expensive modifications due to dimensional constraints; especially modern coil-over independent suspension. BP-51 uses a unique extruded alloy body where the tubes are replaced by passages within the shock absorber body, allowing it to fit into tighter confines such as inside a coil spring.