to do what you claimed and show proof, to prove what you're capable of
A equipe prometeu resolver e agora precisa matar a cobra e mostrar o pau.
The team promised a solution and now has to deliver it and show the proof.
The rural image is literal and evidential: after killing a snake, the person shows the stick used to do it. Figuratively, the second action supplies proof for the first, so the expression means accomplishing what was claimed and demonstrating it.
Falando's Idiom Trainer teaches you to recognize idiom meanings and create sentences using them, so you can speak like a Brazilian.
