"These substances are excreted once more, within hours. "
It is not a matter of excretion, most of it becomes absorbed into one’s system. At the level of the gross body, it becomes absorbed into the nervous system through the blood stream. At the the level of the subtle body, it has an impact upon different subtle energies, particularly prana and apana at the level of the chest and navel.
“And, how does this differ, from other forms of slavery?”
What is “slavery” has little to do with the actions that you are doing, and more to do with the spirit by which you are performing the actions. The right kind of practice is that kind of practice which can act, and yet which is absolutely unconcerned with the fruits of the action. Outwardly - the same action done with awareness look the same as the same action done out of unawareness. But they are totally different. Though the action is the same, one binds, the other liberates.
And to be in a state of slavery is needed in the beginning, because that is the space from which most people are, that is how one has been living for ages. The slavery will have to be used in the beginning and then transformed. The fact is that if you do nothing, nothing changes. A method is needed, not to come to one’s liberation, but to dissolve the psychological hindrances which are preventing oneself from coming to one’s liberation. These obstacles are not simply going to surrender on their own accord - they are a deep part of the programming of the mind and body. Like anything else in life, things will happen because you have created the possibility for it to happen.