the science behind this is that there is a lack of chlorophyll (nitrogen deficient)... add that and see what it does... if it's too far gone, then let it fall off naturally (an indication that a seal was formed at the base). the reason you don't pluck them prematurly are at least two things...
1. you have no idea if you plant is contining to be nitrogen deficient
2. premature braking of the stem makes it suseptable to envirnmental pressures (examples: temperature [freezing], parasites, or infections)