On Yom Kippur, the Cohen Gadol would say אחת, ושלש אחת , ושתים אחת, ואחת אחת, אחת … וארבע
The Imrei Emes explains that the most precious moment for Hashem is when one makes a strong resolve to improve his ways. Afterwards, he may have ups and downs, but it is this first firm resolve which is so precious to Hashem. That is the reason the Cohen Gadol continually repeats achas (one) representing that first moment that he accepted upon himself to improve his ways.
