You want the plants to appear like a tree - with a trunk and a crown - but as the rods that we planted start to root and grow, they will naturally set shoots along the length of the rods. The shoots on the "trunks" of the trees have to be pinched off during spring and summer to keep the shape of the trees while the shoots in the top are allowed to grow. If you fail to do so, you will end up with a very bushy shrub in the pot.
Keeping the twisted trees (from left: Salix x 'Flame, S. 'Americana', S. integra 'Hakuro Nishiki', and S. x acutifolia) free from shoots on the "trunk" is fairly easy as you basically just rub your hand down the trunk while the shoots are small. As you may see on the photos, the different varieties have different habits as to how many shoots they grow and how easily they are pinched off.
The Harlequin tree takes much more work and the variety in the picture, S. koriyanagi 'Rubykins' will have a lot of shoots in the spring and early summer with fewer shoots continuing to show up during the rest of the growing season and the following years.