From http://www.history.army.mil/books/coldwar/huk/ch4.htm
“Force X”
Shortly after the Secretary of National Defense reorganized the constabulary, the government authorized the one truly successful anti-insurgent operation during the first phase of the insurrection — “Force X.” This special force was envisaged to operate deep within enemy territory under the guise of being a Huk unit itself. As such, the force would be valuable in obtaining intelligence and carrying out small unit operations such as kidnappings of Huk leaders and ambuscades. “Force X” was created to take advantage of a period when Huks operated freely in central Luzon but when their command organization was loose and inexperienced.
Philippine Army Colonel Napoleon Valeriano, commander of the Nenita Unit, a special constabulary force that operated in the area of Mount Arayat from 1946 until 1949, selected the 16th Police Constabulary company, under the command of Lieutenant Marana to become “Force X”. Secretly screening his unit for the most devoted and aggressive men, Marana selected three officers and forty-four enlisted men who departed their barracks under the cover of darkness and moved to a secret training camp in the nearby jungle. The camp’s location and purpose were known only to the president, the Army Chief of Staff, Col. Valeriano, and three of the president’s closest staff officers. At the camp, the unit was stripped of issued clothing and equipment, and given captured weapons and old civilian clothes. Using three captured guerrillas as instructors, “Force X” received training in Huk customs, practices, and tactics to help them pass as the enemy.
Each man assumed an alias as well as a nickname, a technique favored by the Huks, and began to live life as a guerrilla.
After four weeks of intensive training and a careful reconnaissance into the area where “Force X” would initially venture, the unit was almost ready to go. To complete the scenario, Colonel Valeriano recruited two walking-wounded from an Army hospital in Manila and secretly transported them to the training camp. At 1700 hrs, 14 April 1948, “Force X” fought a sham battle with two police companies and withdrew with their “wounded” into Huk country. Four hours later they were met by Huk troops, interrogated as to who they were and where they had come from, and were taken into Candaba Swamp where they met Squadrons 5 and 17. Marana convinced the commander of his authenticity (a story based on the death of a genuine Huk leader) and was promised that he and his forces would be taken to Taruc. The cover was working better than expected.62
“Force X” spent two days at the base-camp learning a great deal about local officials, mayors, and police chiefs who were Huk sympathizers and about informants within the constabulary. As they awaited their appointment with Taruc, they were joined by two other squadrons, one of which was an “enforcement squadron” whose members specialized in assassination and kidnapping. On the sixth day in camp, Marana became suspicious of Huk attitudes and ordered his men to prepare to attack the assemblage. Quietly removing heavy weapons (including four 60mm mortars, two light machine-guns, 200 grenades, and a radio) from hidden compartments in their packs, “Force X” attacked the unsuspecting squadrons.
In a thirty-minute firefight, “Force X” killed eighty-two Huks, one local mayor, and captured three squadron commanders.
After radioing for reinforcements to secure the area, “Force X” took off on a two week long search and destroy mission, accompanied this time by two infantry companies. During seven engagements, government troops killed another twenty-one guerrillas, wounded and captured seven, and identified seventeen Huks in local villages. “Force X’s” success did not stop when it withdrew at the end of the operation. Three weeks after the incident at the Huk base-camp, two squadrons stumbled onto each other and, each assuming that the other unit was “Force X,” opened fire. The panic and mistrust that “Force X” put into Huk ranks cost the insurgents eleven more dead from this chance encounter.