The simplest explanation is really simple. The tagalog term sangkutsa simply means “pre-cooking”. Half-cooking is also acceptable.
Sangkutsa is done by adding one to several flavorings to the main ingredients. It can be water only, or water plus salt, fish sauce, onions, garlic and soy sauce. The last step is boiling the mix until half done. The boiling time varies depending on commodity.
There are three reasons why do sangkutsa or isangkutsa. The first one is to remove unwanted food components or a possible toxin and unwanted flavor. The not so popular example is the “kabuting saging”. It is a mushroom that grows only on decaying banana corms. According to my elders, its flavor is undesirable when not pre-cooked.
The next reason is to soften the main ingredient before the flavorings are added. The technique is usually applied to native chicken hen. Pre-cooking such is a long process but pressure cooker shortens the time significantly. Cooking a broiler chicken instead removes the need for pre-cooking procedures.
The last reason is extension of products shelf life for few hours to a day. A dressed chicken bought in the morning might be spoiled already by afternoon but not when pre-cooked. Full cooking it in the afternoon further extend the storage life by the next morning. The affordability of refrigerators voids the need for this method.