Some other people already hit on the topic of how the word submissive is not the best word to use, so I'll go on from there.
In my opinion, feelers by nature will try to please others. One of the most common ways to please others is to not be forceful. I feel IxFPs are the least forceful of all people. They will not (for the most part) force anything on anybody, but allow the other person to make the decision. By having the other person make the decision, the decision maker is happy. Because the decision maker is happy, the other person (IxFP) is happy as well. There is a flip side to the coin. If the decision maker tries to force his/her way on the feeler, the feeler will not be as happy going with his/her decision. In the end, the feeler will try to avoid the forceful person, so the conflict of that person being forceful doesn't happen.
It isn't so much IxFxs like to submit, but they are happy allowing others to take the lead. The only problem I've experienced is everybody needs to state what they would like to do every now and then. If you don't state your wants and needs at a healthy rate, then you can bottle up some negative emotions. What I mean by this is feelers (in general) will sacrifice their wants and/or needs for the wants and/or needs of others, so there can be peace in the group. If the group or person doesn't take the time to consider the feeler's opinions because they don't state them, the feeler can end up bottling up negative emotions about those people because they don't take the same amount of time to consider the feeler's feelings as the feeler would theirs.
In the end, I've notice two kind of reactions. The xxFJ (in general) will end up exploding in a huge rage shocking everybody and even scaring them. It is there that something finally gets done, but the damage has been caused and could of been avoided if the xxFJ had let his/her opinions known earlier. The xxFP (in general) will end up never releasing their anger at all, but allow it to dwell inside of them. This causes the anger to turn into bitterness, which is by far more damaging than any other kind of emotion. When a person is bitter, they will always show some kind of negative emotion towards the person they are bitter against. The person who is on the receiving end of this will have no idea why this person has turned into such an negative person. The only cure for this is for both parties to see the problem and come up with a solution together. Talking about one's issues (regardless of the personality type) is always one of the best methods of solving the problem.
Overall, some people are more forceful by nature, while others prefer to back down by nature. xxTJ forceful, xxFP back down, xxFJ gentle forcefulness, xxTP just plain quiet but stands strong.
Man I can get very wordy. I hope this makes since. Ok ... I'm done now.