To change to a new profession or specialization later in life without having formal training in it; to enter a line of work from a different background.

literally "Cheng Yaojin ambushes the enemy mid-way"; used to describe someone showing up unexpectedly and disrupting a plan or causing a situation to go wrong

someone who shows up unexpectedly and disrupts a plan or situation. This expression refers to the general [[程咬金|程咬金|cheng2 yao3 jin1]] who is famous in folklore for launching sudden ambushes