What is dual mode adaptive landing gear system?
From Testing and analysis of dual-mode adaptive landing gear, taxi mode test system for YF-12A the purpose was to obtain a very progressive rate to keep the gear from bottoming out on landing, but then move to a flatter rate to minimize ground shock when taxiing.


Two air reservoirs of differing capacity and pressure were valved to the hydraulic struts. The smaller reservoir was used for landing as it reached higher pressure faster with a given amount of fluid displaced by the compressing strut. After landing the strut was switched to the larger reservoir.
My reference provides a description of the test configuration as being similar to the actual construction, which it says is in its Reference 1 which I was unable to locate.

In any case, it is simply an additional air (now nitrogen) cylinder connected to the service port of the strut, a somewhat higher pressure is used and there is no valve since they were only testing a single mode, taxiing. The smaller reservoir is likely just the self contained strut air reservoir as in the unmodified design.
Unmodified strut

Automotive shock with reservoir to show test configuration (this was not used, it is not even a strut)