Topping is a method of high stress training that's typically done when the plant is about 21 days old. When a plant is topped, the grower clips off the main ("apical") stem about 1cm above the fourth or fifth node, growers choice. If you don't remove the apical stem you will get a "Christmas tree" looking plant unless you bend the apical stem over and tie it down or weight it down.
When the apical stem is removed, the growth hormones that would have gone to the apical stem are instead sent to the stems at the fourth/fifth node so those branches grow significantly larger than they would if the plant is not topped.
If your plant is in flower and is too tall, growers will "supergroup" stems. As
@Phytoplankton advises, you can uses pliers to "crush" the stem but only enough to allow the stem to be bent over. Another option is to "roll" your fingers over the place where you want to make the bend. Similar technique, same result.
Back to topping - topping removes just the apical stem. There's another HST technique named "FIMming" where the cut is made a bit higher up the stem. I don't have a picture handy to describe a FIM. Do a Google search on "images FIM" and…you'll get the picture.
I top and supergroup photos and autos. It's all cannabis.