Specifically here I believe he is referring to the pulse width of the square wave, like you would see on many basic oscillators. A 50% setting would have the wave high for half of the cycle, then low for the other half (111110000011111000001111100000 etc). A 1% setting would have the wave high for 1% of the cycle, then low for the remaining 99% (something like 100000000010000000001000000000). This is still a 440Hz periodic signal, just with a somewhat different timbre.