If you need help, the application can give you hints by pressing <Ctrl-F1>. This feature marks those cells by blinking in red or orange, in which only one number can be entered and the problem remains valid. If the cell flushes in red, the value for the cell is obviously, otherwise (orange flush) indirect hints must be taken into account to find out the correct value.
Another nice feature is to get information about the current cell by pressing <F1>. As shown in the screenshot on the right side, the direct and the indirect blocks for the current cell are displayed in a popup window.