TK has properly defined pinching for you. However, I want to take issue with your
plant encyclopedia. As with most
plants, pruning (cutting off a portion of a stem or stems) is more appropriate than pinching.
Following a Poinsettia's flowering period, you will probably observe that some new green-leaf growth is emerging in one or more places on the lower portions of the stems. It is best to prune each stem back to a point just above the lowest new-growth point on the stem. This will leave the Poinsettia looking a little sparse initially, however Poinsettias are rapid growers and will soon fill out. The aim here is to have a shorter more compact
plant with lots of healthy new growth that will grow through the
spring and summer.
I have written an article on year-round Poinsettia care that I will send to anyone who sends a request to me at wcreed@HorticulturalHelp.com
Will Creed
Indoor landscaper
NYC