In the same way you would isolate a finger. If you open your hand and extend all of your fingers you can have your index finger move relative to your other fingers by curling your index finger. Alternatively, you could curl all the other fingers.
Then the relationalist would use the same description—placing relative motion as the more primary thing that exists—as discussed earlier.
Yes, in both cases there is relative motion but, as discussed, we can distinguish between the two scenarios on a number of bases really, but particularly when factoring in the observers freedom of choice. In one scenario the observer chooses to move their index finger while in the other they choose to move all fingers except the index finger. Both scenarios give rise to relative motion but there is a more fundamental explanation as to why there exists relative motion in the first place, namely one of the two distinguishable scenarios.
A relationalist also distinguishes between the two scenarios above, as the relative motions induced in the two scenarios are different and mutually exclusive. A relationalist would hold these distinct, mutually exclusive relative motions as the more fundemantel thing that distinguishes the scenarios, and does not need further explanation in terms of some other sense of motion.
And how is the relative motion induced? In the first scenario the index finger is moved while in the other, all other fingers are moved. It is this action of moving the specific finger(s) which causes the relative motion making it ontologically prior.
And how is the relative motion induced?
In the first scenario the index finger is moved while in the other, all other fingers are moved. It is this action of moving the specific finger(s) which causes the relative motion making it ontologically prior.
Only according to the substantivalist. According to the relationalist, in the first scenario there is relative motion between the index finger and the environment. In the 2nd scenario there is motion between all fingers and the environment. These alternative relative motions are ontologically prior.