In this reaction ester is reduced to aldehyde. Such reduction can not be achieved by reacting reducing agents like LiAlH₄ because it will further reduce aldehydes to alcohols. So, in order to stop reduction at alddehyde, ester is reacted with bulky reducing agent, Diisobutylaluminium hydride (DIBAL) which works with same mechanism as that LiAlH₄ do. It also add hydride to carbonyl group. The reaction is shown below,