Input the following command on your keyboard before selecting either New Game or Continue
(I J L I J L K) if done correctly, you'll hear Valvatorez say "Sardines" then you select either New Game or Continue and select a file.

After inputting the command and selecting a file, it will grant you access to a lot of features you get later on in the game along with hidden content, from what I've been told you won't lose out on any items or stages besides the starting tutorial stages.

This is my first proper guide on Steam, so I hope it helped and it was easy to understand.