This is from the 2005 ARRL handbook, It also has been in most of the earlier handbooks. It is used to inject a signal in this case a 10Khz square wave that is dirty and rich in harmonics into a circuit. The theory being that it signal will pass through passive components that DC will not, like capacitors and coils. It is also very handy to trouble shoot audio sections in radios. The injected signal can be heard in the speaker, just keep backing up from the speaker until you do not hear the signal any more.

My old signal injector was an old beast, and started to act up. Instead of repair it I decided to build a new one that would fit in a Altoids size box. It is built using Manhattan style construction. A standard RCA plug allows use of one of my standard bench probes, or a cable. I did not add a switch to the circuit, as I just instal the 9 volt battery when I use it and close the lid.