Not sure how this thread went from where can I buy coilovers to "my shock is better than yours" but along the way someone wanted to know about the Bilstein ACV. I am not here to make claims of whose ir bigger or better rather just here to answer a question and leave the proof to the end users.
The basic concept is that a top hose coilover loses stability at higher compression forces unless there is a line restrictor or sifficient gas pressure to maintain stability. So even though the compression valve stack is setup to deliver increased damping (teired valve stack) this increased damping will only occur with sufficient counter pressure. When the compression force overcomes the gas pressure then instead of the oil going through the piston it will be shoved into the reservoir. If a greater amount of oil than the shaft displacement goes into the reservoir there will be cavitation. The ACV works in time against the main compression valve stack so as forces increase the coilover is actually able to engage the high speed valving already present. Moving the reservoir port to the bottom of the shock can gain this compression effect but it also destabalizes the rebound and since a coilover has a spring wound around it porting from the bottom is not realistic. Basically the ACV allows a coilover to gain the stability of a botom hose without sacrificing rebound stability.
There are some other uses, configurations, and effects related to the ACV but thats a story for another day.