Alzapua is a flamenco technique involving the thumb. the term comes from the word alzar which means to raise.
It is usually played as a fast flourish of notes in quick sequence. The thumb plays all notes using down and up strokes. Played accurately and fast it can sound fantastic as a high point of a falseta or phrase.
Here are 2 Alzapua exercises, the first uses triplets and the second is in groups of 4. Make sure to practice it slowly and in time with a metronome.