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"Power domain" is an expression borrowed from the world of aeronautics, and this type of graph is of an undoubtable usefulness for benchers.

In aeronautics, any aircraft has its own "flight domain"; this means that its speed and range varies according to its altitude. The curve in question draws an internal surface within which one can be certain that the aircraft wil be able to fly. As the altitude decreases (in the lower part of the diagram) so do the speed and range, and vice-versa. Speed is on the abscissa and altitude on the ordinate axis, the limits being the maximum altitude and the maximum speed (left).
The higher you fly and the farther you can go, and the lower you fly the lower will your speed be, and shorter your range.
The same applies to an athlete's strength. The heavier the weight the less numerous will possisble reps be. This gives a curve defining a surface akin to that found in the "flight domain" of an aircraft; this is the "power domain" that is proper to each benchpress athlete.

You'll quickly realise that your personal curve is far from being linear, and areas will appear showing the domains where you are powerful (peaks), and those where you are weaker than could be expected (when the curve goes lower than its own overall tendency). These differences are related to each one of us' innate distribution of our different muscular fibre types, between the quick and the slow-twitch ones (in other terms, those responsible for either endurance or sheer strength).

The interest of knowing one's own power curve lies in being conscious of the type of sets that need to be improved. Because a 1 rep maximum improvement is not only related to your ability to do heavy 2 or 3 reps sets only. You can just as well improve this 1 rep maximum via longer reps, between 7 and 8 for example. Because any improvement at any level of your "power domain" curve, a peak going above your average curve, either at the level of short sets (1-3 reps) or longer ones (6-7 reps), will certainly improve your 1 rep maximum ability.

Hence there is no need to concentrate on heavy and short sets only. If you identify the "areas" requiring an improvement, this will aid your progression. For example, in the example shown above, one can see that 10 reps with 120kg are done, but only 9 with 117,5kg (because this must be an outdated record). Therefore, there lies a progression margin to be explored. This progression can either be done through an increase in the number of reps at the same weight (117,5kg) (green arrow) or thanks to a increase in the weight used for the same number of reps (7) (yellow arrow).