There is a significant difference between Derwent's Inktense and watercolor pencils. When you add water, Inktense produces ink not watercolor paint. Once this has dried, the ink is waterproof rather than remaining water-soluble with watercolor.
This means you can additional layers to your painting without disturbing what you've already done. This will be a fmiliar method if you are used to working with Acrylics or Inks. It is fundamentally different to working with watercolour where overworking can result in muddying the colors by overworking, but with Inktense once it's dry, it's dry.That said, it's important to note that if you don't 'activate' all the Inktense pencil the first time you apply water, you may have some pencil left that will dissolve the next time you apply water.
As well as able to use Inktense in pencil form it is also available as a block. Watercolour Inktense blocks combine the brilliant colours of Inktense pencils with the freedom of blocks making it easy to cover large areas really quickly. You can use these watercolour blocks dry or add water to create deep, intense colour washes. Once dry Inktense becomes permanent and you can work over the top of it so it great for layering and for use on silk and cotton!