Communication is everything! Your partner’s level of effort may not always (ever?) be apparent. It is possible for them to prostrate themselves on a climb, whilst you cruise along thinking that the going is easy. My Sweetie and I thus declare (negotiate?) a consensus level of effort, on a scale of 0 – 10. That level might remain constant for a good while on a flat road or on a steady climb or on a steady descent. On the other hand, that level can change every few seconds when encountering lesser dips in the road or a highway overpass. Momentary shifts in effort allow maintenance of cadence, resulting in fewer gear changes. Declaration of “Zero” is useful when one person needs to shift saddle position or some such.
When we do change gears, the person at the controls declares, “Shift!” so that their partner knows to let up on the pedals until the shift is executed. With practice and team confidence the let-up probably only lasts about 0.3 seconds.