The basic idea is that you blend to colours into each other while still wet on the mini. This works well for basic shading and highlighting on larger areas, prefarably smooth ones (but not necessarily), like cloaks or robes or similar. You put on one colour first and then you put the other on next to or on top of the still wet paint of the first colour. Then, by moving the brush back and forth you gently mix them together until you have a smooth transition.
As you probably understand, this takes quite a bit of practice to be able to do well, but once you master it it\'s a very fast way to create smooth blends.