This achievement requires that you swap two tiles at once to create a new snap. First you need to create a snap. Then, come up with another snap you could make simply by swapping a single letter you have with one already in the snap on the board. For example, in the image below, PILLS is locked onto the board to start. The snap SHARE is created. In your rack, you also have a P. By dragging the P and dropping it in the place of the E in SHARE, a new snap, SHARP is created, unlocking this achievement.