If writing is made on a piece of paper which is resting, either directly or indirectly, on another piece of paper then indented impressions of the writing will be created in the lower piece of paper. A typical example of this is in a pad of paper.
Clues left by indentations
Indentations can be used forensically for a number of different purposes. They can provide intelligence information such as names, addresses, telephone numbers and email addresses which are not visible to the naked eye. In more complex cases the order that writing and indentations were made may be deduced providing significant information regarding a sequence of events. For example an entry in an old diary may have been made much more recently than the date in the diary indicates in order to present a misleading scenario (e.g. to present an apparent alibi for a crime).
Indented impressions can be visualised using ESDA, by oblique light (light shone at a shallow angle) or by creating a cast of the surface of the document. For normal paper ESDA is usually the preferred method as it is the most sensitive and provides a permanent 'lift' showing the indentations. However if the paper has been wetted (e.g. the document has been treated for fingerprints using ninhydrin) then ESDA won't work and one of the other methods will be required.