When the piston reaches the last 20% of travel, it enters the Adjustable Hydraulic Bottom Out (AHBO) circuit, where it pushes the oil through the Hydraulic Bottom Out port, the size of which changes based on where the AHBO adjuster is set. When the adjuster is all the way closed, it increases the pressure building in the AHBO chamber to create more damping and make it harder to bottom out as the shock moves through the stroke. If the adjuster is all the way open, there is no additional bottom-out support, so it’s easier to reach full travel. For a more thorough look at the Hydraulic Bottom Out design, check out Rear Shock Hydraulic Bottom Out.
The chart below shows the initial stroke of a 2024 Vivid with the TouchDown damper (red lines) and a 2024 Vivid, same tune/same shock with a standard damper (black lines). In the first 10% of travel, both at medium shaft speeds (upper lines) and slower shaft speeds (lower lines), Vivid's TouchDown damper has a decrease in compression damping, meaning that it requires less force to initiate travel for both shaft speeds in comparison with the standard damper. Then, there's a smooth transition as the piston moves into the main compression circuit and utilizes the shim stack. An important takeaway from this chart is that the transition from the TouchDown circuit to the main compression circuit is seamless—there is no effect on the rest of the stroke.