DS cartridges manufactured by Nintendo are set sizes, 8Mbit, 16Mbit etc so there is often blank space on the cart which is not being used by the game (00000's at the end of the ROM). Using a ROM trimmer you can cut off this unused space from the end of the ROM and make it smaller (how much smaller depends on the game), saving space on your SD card and enabling you to fit more games on. You should have no problem playing trimmed games on your EDGE card.