Masked sponging is an old technique for creating backgrounds. Using different shapes to mask off an area gives a unique and polished look to the finished project. These cards are made using different basic shapes. But, keep in mind that other die cut shapes can give great results. Some outline shapes that comes to mind are hearts, leaves, and stars, or other basic shapes like diamonds.
Starting with two panels the same size, one is die cut with a basic shape. Then the one that was die cut is temporarily laid over the other panel for the sponging. You can keep the masked sponging simple by using only one color ink like the bottom right card. Or you can blend two or more colors for a different look. When you’re happy with the ink sponging, remove the mask and stamp or add a silhouette-like die cut. Add a sentiment to complete the front panel. Stamping or layering an image that extends beyond the sponged area is more pleasing than if the image was confined within the sponged area.
Another example of masked sponging is found in this post, where an area of the front panel is taped off to create a horizon on a scene card. Or the card in this post, where the frame die is left on the front panel as a mask to sponge inside, to create the white frame around it.