Stain pretreatment contains special enzymes to attack the protein of the stain. It attempts to break down the protein goo into water soluble products. The enzymes take time to work. So pretreating and keeping the item in a plastic bag(?) so the pretreatment does not dry out will allow the enzymes to work. Work the fabric a little every now and then to get fresh enzyme to the surface of the stain. Wash out normally. This not unlike the basic idea of the gasket mold cleaning procedure. Pretreatments may also contain mild organic solvents. This removes the grease or oil from the protein molecules so the enzymes can get direct contact.
Detergents on the other hand attempt to surround the mildly ionic soil with ionic chemicals and try to lift it off the surface. They also cause the oils and greases to dissolve in the water so they cannot hold the soil in place.
Google “micelle” for extreme technical explanations. Stains are large gooey proteins that do not dissolve easily in water so detergents don’t do much to a stain. The big molecules need to be broken up so they can be dissolved in water. This is true of even runny liquids like red wine. The “red” is a large protein molecule. It is held in solution in the bottle by the alcohol content (organic solvent). Once the red wine stain has dried the alcohol is gone and the protein does not want to go back into water solution during a normal wash. Pre-treatment enzymes are required.
If the stain has penetrated the pores of the fabric it may take a really long time for the enzyme molecules to get to it. This is an important consideration. Especially for dried stains. Washed and dried stains are impossible to remove because the long protein molecules have come into contact, linked and polymerized to the other long chain protein molecules. This makes them too big to dissolve. Enzyme cracking of the molecules with a long pretreatment soak is a method of breaking them down to be dissolved.
A oxybleach product may reduce the residual stain. The bleach can break down the remaining stain proteins into smaller molecules so they have less color and may become more soluble to enable it to be washed out
If the stain is just a thick grease or oil use the SuperShock Home Recipe formula of vinegar, salt and dishwasher detergent to make a pre-soak using hottest water allowed by the fabric. soak/agitate a long time. Wash.