The Try button is very useful for hard Sudoku problems. If you get to a point where you can't make progress without trying one of, say, two options in a cell, click the Try button. Then try one of the options. If it turns out to be right and you solve the puzzle, then that's it. But if you reach a broken position, click the button again (which is now called Untry). It will restore the position to where you were before you tried the failed option. Now you can try the second option.