A crystal spotter is just a simple oscillator which will emit a very low level signal when a crystal is attached. It's used to locate the crystal's operating frequency in a receiver, when the receiver is not slaved to the same oscillator as its companion transmitter.
It's very useful with the Bayou Jumper, which has a wide pass band regenerative receiver, completely independent of the transmitter. If you're calling CQ with a Bayou Jumper you need to know where on the receiver dial to look for replies, since most operators using modern VFO controlled transceivers will zero beat your signal. It also helps to find out if there is other activity on your crystal frequency so you won't QRM them.
The spotter does not have to be connected to the receiver, just placed nearby. Enough signal leaks out to be easily heard.
BTW - whenever you are the "other op" trying to call and work a Bayou Jumper, remember that poor fellow has a regen. It's wide as a barn door, very sensitive, but he has to manually throw a switch and then perhaps tweak the tuning a bit to get back to where he was. His or her tuning may be as much as a couple kHz or more from a dead-on zero beat. So the typical "drop your call once" won't cut it.
Sending "callsign callsign de mycall mycall AR" is makes it much easier for that poor schmuck :^))
Back in the "spy days" the home station would call the outstation for several minutes to allow time for the hard pressed agent to get everything tweaked up.
second BTW - because of all this, a Jumper-to-Jumper QSO is a real challenge and an accomplishment to be savored.
Bayou Jumper almost WAS.... -AK