Pitch raising involves adjusting the tension of all the strings in a piano when they have dropped significantly below the standard pitch.

If a piano has been left a long time without being tuned or if it has undergone some kind of trauma, it may need a pitch raise instead of a standard tuning. 

A pitch raise is very similar to a tuning but requires a far larger level of tension to be added back into the strings, which is therefore a more complicated and time-consuming procedure.