First, what is power? Power is leverage. Power is influence over the decisions of others. The person with the most influence has the most power. Now who has the most power is rarely a simple measurement, however a good guideline is: who has the least value at risk? The person with the least value at risk is the one that can walk away from the situation the easiest, the one who can most easily decide to do something else. However, the power position isn't determined just by how much is at risk, but by how much is perceived to be at risk, because that perception is what people will base their decisions on. So the person who is in the power position is the one who perceives that they have the least at risk; the one who thinks they will be the least affected by the influence of the other person's actions on their existence.
The whole "submissive has the most power because the submissive can end the scene" is a myopic way of looking at the situation which gets endlessly perpetuated largely because it boosts the comfort level of otherwise nervous (and generally green) submissives. It seems at first blush that the submissive has more power because the idea always gets brought up in the context of the dominant wanting to 'go too far' and that the dominant will stop when told. Yes, the dominant is supposed to stop, but that doesn't mean that it will happen. The dominant still gets to make a choice, while the submissive does not. The submissive isn't demonstrating more power in that situation, but asking the dominant to weigh the perceived consequences of continuing vs. the dominant's desires. If the dominant decides that the risks associated with ignoring the safeword are less than the continued enjoyment of what s/he is doing, then the submissive just might find their safeword ignored. If the submissive had the guaranteed ability to bring a halt to whatever was happening at any time and without consequence, then the 'submissive' would be in the power position because s/he has the least at risk, thus s/he would not actually be submitting.
The whole point of a d/s relationship is that there is a power differential in the dominant's favor. A dominant that is doing their job correctly won't have the submissive thinking that s/he will be the least affected by refusing to comply. If you think you are in the power position, then you certainly aren't thinking you are the submissive one in the relationship.