Don't trust the "Usage"! You have to do it with a space between the ! and the letter. Like "! w banana". It does work!

Not working at all ... tried with "w banana", "!w banana" and others bangs availables (from github project) and always Google Search page opens with the url "https://www.google.com/search?client=firefox-b-d&q=w+Banana". Btw, You cited that the search opens the results without leaving the current page ... completely wrong! So useless for now