That is a good question, and i shall try to answer it. I have experience as an online Mistress, and i have my views on how it SHOULD work.
First of all, a Domme/sub relationship, is a relationship. The Domme and the sub do this with each other because they choose to, and as far as i am concerned, ONLY while they BOTH choose to.
Second, it is kind of like acting, you pretend to resist, pretend you dont want to. Or for some fun, you set it up do do it, in a way you dont want.
And there is always the aspect of setting it up to where that you get something out of not doing it, but being required to hold it too long. So think of this, your Domme will do something for you that you like IF, you can hold it long enough. But of course you will be given lots of water and diuretic beverages to make you wet sooner, and possibly even something to increase the odds of messing. Then when you are certain you have less than like 30 mins, you get to find out you need to make in 2 more hours.
This is honestly not an easy topic to advise and teach, without actually partaking in some way. I learned by doing online. I have worked with a number of those (both men and women) who enjoy the desperation of having to go badly so that is always a part i like to add.
Plus with a D/s relationship there is always the aspect of trust, you have to trust your Domme to stop when you ACTUALLY want to stop. A safe word is used for that. And a safe word is a word that will most likely NOT come up during the session. (like in an episode of Family guy Peter and Lois are starting a little D/s play and Lois says the safety word is banana) Can be any word you know you can remember. Some like to use their fav food or color.
The trust side also comes into play a lot online, like trusting that when you tell a sub to do something online that they actually do it.
I am not sure if this helps any, but atleast i have given you some info that may help.
Laterz Janessa