Let’s say our offense is currently 30% designed run, 30% designed pass, and 40% run/pass option (RPO). That looks pretty balanced, and assuming that on RPO we run 50% of the time, that gives us a 50/50 run/pass ratio.
But if the defense sells out to stop the run, then the RPO suddenly becomes 100% pass and our run/pass balance shifts to 30/70.
if we want to be 50/50 balanced then we have to change the playcalling to be 50% run, 30% pass, and 20% RPO. That instantly turns us into a power spread to run team without changing the playbook or personnel.