This string of characters: 如力士屈申臂頃
作是念已，如力士屈申臂頃，於兜率天沒，現於佛前，稽首佛足


他有了這個想法後，有如壯漢彎曲再伸直手臂的頃刻，在兜率天隱沒，並出現於佛陀之前，頂禮佛足。

After he had this thought, as fast as a strong man bent and straightened the arm, disappeared in Tuṣita Heaven, appeared in front of the Buddha, and bowed the Buddha's feet.

如力士屈申臂頃 describes the speed is very fast or the period is very short.
屈 and 申(伸) are two different movements.
屈臂 means to bend the arm.
伸臂 is to straighten the arm.
A strong man does not need much time to do these movements.
Therefore, 如力士屈申臂頃 can be interpreted as "as fast as a strong man bent and straightened the arm".