You may notice a certain nautical theme that I use to label the techniques that I use…   let’s begin with Longitude and Latitude (L&L). You are already familiar ,no doubt, with this technique. Simply using existing fulfils to discover new permanent ‘candidates’.

Consider the grid…

You can see that in the 6^th square the only possible position for a 1 is the black cell; the longitude (rows) and latitude (columns) define this.

This is possibly the simplest of examples, however there are occasions when resolving this cell would lead to identifying a 1 in another square. Alas, in this case, this cell in square 6 offers no assistance in using L&L to find other 1s.

However, finding the 1 in this cell does, of course, aid finding other fulfils in this square, row and column.

The diagram I have used here is from RTR Sudoku. From the Tools Menu you are able to highlight any of the digits, 1 to 9. You can also use Shift+F1 etc. (although I seem to have use Shift+Fn+1). Pressing  Shift+F1 turns off the highlighting.