"I'm in charge!"
This is a realization that some submissives have from the beginning and that hits others like a thunderbolt.
The Dominant has Control, but the submissive chooses to be under it. The submissive chooses to kneel, and sets limits within which the Dominant controls what happens. In time, trust grows and the scope of that control will grow with it.
In a sane, safe and consensual relationship, a submissive has control over how far she submits and within what limits. It is confidence in this control, which is expressed as trust in her Dominant, that allows her to truly submit.
This includes having control over:
safe sexual practices;
stages in the transfer of power;
the involvement of witnesses or participants;
the creation of photographed, videotaped or other evidence of activities.
A submissive also has the right to be well and truly used, to have as much power as she wants to give taken, and to be taken on explorations of all the realms of experience that this power exchange will open.
A Dominant can also balk at any crossing of a boundary in these regards, and is just as capable of having limits physical or emotional. And if a Dominant does have needs or limits that may impinge on the relationship, He must disclose these factors up front. Whether they be limits on the extent of commitment He can make, a need to acquire other submissives or the need to work towards commitment and focus on monogamy. One person's limit is another's prize.
Beware anyone who starts any statement with "a good submissive/slave would ..." and completes it with anything that would not be applicable to Dominant and Master as well, such as "be truthful, honest, passionate, caring and committed". A recurring issue in this regard is the Dominant who informs a slave that He desires to have another slave - and responds to her concerns with the answer "if you are a good slave you will accept this."
A "good slave" is good within her limits. This has nothing to do with whether or not she accepts particular forms of play or types of relationship. She should never confuse her true worth with how well she meets one Dominant's expectations. This is easier to do than it seems, for part of what a submissive wants is the affirmation of praise and accomplishment in a Dominant's eyes; manipulative people seek to abuse this desire by convincing a submissive that surrendering on such issues is a test of their quality.
Potential partners need to have similar expectations and D/s typically includes exploring a variety of sexual practices. Whether it is sexual acts such as fellatio or anal intercourse or SM acts such as discipline and bondage, everyone has the 'buttons' they want pushed, and some they need pushed. Part of the fun of D/s is realizing just how many buttons there are and the combinations of effects they can produce.
These factors decide whether or not two people are suited to each other, not whether or not they are "good" submissives or slaves or Dominants or Masters.
If a Dominant announces to a submissive after many months that she should accept Him taking another slave; she is not being a bad slave to refuse, He is being a poor Dominant to leave telling her that he is not monogamous so late. Everyone should tell each other early in a relationship about their longer term needs and expectations.
The success of a relationship depends not on the totality of a submissive's surrender or the completeness of a Dominant's Mastery, but on the ability to discuss, to amend and develop, to grow and to find mutually desirable ways of expressing needs and exploring desires. To develop together what erotic power exchange will mean for you.
In time a submissive can trust her Dominant to make the right decisions in these regards. They will be the right decisions because with enough time He knows her well enough to make the right decision about her, and His temperament is such it will be the right decision for her as well as Him.
A submissive has the right to trust her Dominant, and to have that trust based on open communication, honesty and gradually accumulated experience. No Dominant can expect a submissive to blindly entrust Him with power over her mind or body. No sane Dominant expects this trust blindly or immediately - not only is it unreasonable, but it cheapens it.
This trust is a precious trust. There is a wonderful warmth of feeling a Dominant has knowing they have received the gift of trust and submission from an intelligent, perceptive and strong human being because they believe you are worth it.
As someone once said to me of their choice of partner,
"I chose him not just because I thought he would always do what he thought was right, but because I knew it would be right."