During therapy, he slowly began opening up about his childhood trauma.

opening up 是动词短语 open up 的现在分词形式。open 本意是"打开"，up 表示"完全、彻底"。 open up 作为短语动词，意思是"敞开心扉、倾诉心事"，指愿意分享内心深处的想法或感受。