a pinch

In the Cannabis Growing industry, in Canada anyways, pinching refers to removing the growing top parts of a Cannabis plant (called the apical meristems in biology) , and often the grower uses his fingers to remove the young shoots of the plant, thumb & nails as if to pinch something that's why they call it to pinch to chop off the young shoots of the plant, so that the continuing growing part of the plant will divide and later produce more product, more Cannabis smoking material, according to professional growers.