The pen comes with a fabric folding solution that makes fabric pliable and easy to fold.
This is one of my favorite products to piece my blocks with and it makes a HUGE difference in accuracy. The pen gets filled with water and then 4-5 drops of the included softner is put in too. Before pressing a seam, run the fabric folding pen down the seam and then press as normal.
The seam will lay crisp and flat and PERFECT! It has changed the way I piece and you will love it too. At first, I was skeptical and thought it would be an unnecessary step but I promise you that your accuracy is going to increase.