The salt water flush has to be done when your stomach is empty.
Otherwise the saltwater will combine with the food and not act as a flush.
Keep in mind that the only reason that the salt water flush passes through your intestines (and is not absorbed) is because of the specific gravity of the salt water mixture. When the specific gravity of the salt water is the same as the blood then the kidneys will not absorb it. The 2 teaspoons of salt per quart of water creates a mixture that is the same specific gravity as your blood.
If your stomach is not empty or even if you have liquids already present in your stomach (you just drank a cup of coffee), then the salt water mixture will combine with the other liquids and it will change the specific gravity of the salt water mixture thereby allowing your kidneys to absorb it instead of passing through your intestines.