From Zelda Wiki, the Zelda encyclopedia
In The Adventure of Link, the Up Thrust is a skill taught to Link by a Hyrulean Knight in Darunia Town. Since the door to the Knight's house is locked from the inside, the young hero must use the Jump Spell to jump from roof to roof and eventually enter the house by going down its chimney. At the end of the basement, the knight teaches Link his new skill upon talking to him.
When Link uses this skill, he leaps into the air and thrusts his sword straight up, attacking enemies directly above him. This is especially useful against flying enemies such as Aches, and also for clearing out blocks in Palaces after getting the Glove.