The real dom is actually the sub. 🙂 Sorry to ruin it for you. 🙂.
Actually, NO!
This is a common misconception that is constantly being perpetuated especially to new and/or curious people about BDSM.
The sub is not the "true" one in control. Why? Because, the safe word can in fact be used by ALL participants even though it is almost always directed at the sub/slave/bottom role. Because, the one in the dom/top role has a unique way of expressing or using the safe word without actually using it. Like verbally checking in with their partner(s) without safe wording before the casual questions. And also, ending the scene or play earlier than anticipated again without actually using the safe word first. <- Yah, I know real sneaky of them!
As for when a safe word gets used, depending on those involved (of course!) a large portion of the time the dom/top will check in first before completely jumping into "end scene now!" mode, because a simple adjustment maybe all that is needed instead of a complete scene stop. Or they may need to know where to start ending the scene for instance if bondage is involved untie or emergency cut off?
Now, when the time comes for the scene to end or something because, of the safe word sure the Dom/Top can always ignore it, but I sincerely doubt that their play partner will be returning or will be providing any form of a positive referral for the them. This also, shows a severe lack of common courtesy, manners, respect, or concern for another person's health and wellbeing. Remember, a safe word is not some sort of magical word that will make any and all dom/tops obey no matter what! They can choose to ignore it and those that do or joke about ignoring it (especially during the scene) are not safe to play with. (And I'm not say that those that do make those jokes are not safe to play with, but they do need to understand that not everyone is going to enjoy their jokes or find them reassuring in any way.)
Also, there is this thing called "topping from the bottom" which is when the person in the sub/slave/bottom role uses the safe word to control the scene, basically not using it for what it is intended for.
What is a safe word for then? To notify another person regardless if they are participating or not in the scene that the user is in some form of duress. Could be unintentional pain that is not considered part of the scene, maybe an unknown mental "trigger" was tripped, etc. It is usually highly encouraged for those who have little to no experience playing with each other or if the sub/slave/bottom is vocal with using phrases like "no," "stop," or even crying. Because, these can make it difficult to tell when the person is actually in duress.
So basically, a safe word is a safety-net not a choke collar for the dom/top with the sub/slave/bottom controlling it.
Or
It's kind of like dialing 911 or the police. It is meant for emergencies not for manipulating another person when there is no emergency.