The Jab is a punch thrown from the lead hand, which for our system of Thai boxing is a left lead. The punch is the closest weapon to your opponent and can be used as a range finder, a harassing technique, or a visual obstruction to set up your follow up. Most people are not powerful enough to jab someone unconscious, but several fast accurate jabs can wear down an opponent and expose openings in their defense. It is often thrown in repetition or combination with the cross and hook. The jab is thrown off the left lead, with force starting from the rear right foot which is up on the ball of the foot. Thrust from the calf and leg muscles push the whole body forward quickly. A slight torque of the hips, back and shoulder add  rotational force as well. Finally the extension of the arm muscles add the remaining zip to the jabs force punch thrown just with arm strength is considerably weaker than one with a little body behind it.
The Right Cross is a powerful linear attack. It also begins at the feet; with a push from the rear foot; followed by a torque of the hips, abdomen and back, and shoulders; finished with an extension of the right arm. As with the Jab, the rear hand guards the opposite side of the face and chin, while the right side/chin  is held below the line of the shoulder.
The Hook  is a circular weapon used to strike targets that are accessible from side. Again, force  (rotational this time) is generated from the ground up: I.e. foot, leg, hip, back, shoulder, to arm. The hook can be thrown; close to the body with the arm bending in, mid range with the arm even with the elbow, or wide with the elbow opened for greater reach