its called reverse logic or negative logic. instead of testing something to be true you are testing something to be false.

in your function it could be written like this

if contains.usedword(whatever){ return} else{ do something special }

if you need to do something only if the function doesnt return true then you can eliminate the if block and leave else block cos thats what you are interested in.

as you cant use else block by itself then you are checking whatever is in else block i.e. do something only if the function returns false.

if !function_returning_true{ do something special}

I was struggling with understanding negative logic back when i started C programming but after getting the concept i was using mostly negative logic in my code.