I am reading 'Guide to the Practical Study of Harmony' by Tchaikovsky. On page 63, he says
to pass from one key into another it is not sufficient merely to employ a number of chords not belonging to the main key—we must clearly establish the new key; that is to say, we must insert a chord, that can belong only to the key we wish to reach and which, forming the bridge between the two keys, clearly defines the new from the fold.
What exactly does it mean to "establish" a key?
To provide an example, I recently wrote a song involving only two pitch classes C and G. I wrote this song explicitly to study the nature of key. Interestingly, I could instantly change the key of my song by inserting a pulse into the background. Specifically, if I played an E note over and over again, the song sounded major, but if I played a E flat, then the song sounded minor.