Basically, the crystal checker outputs the oscillator sine wave through the BNC but it also have provisions to an output meter in the form of a plus and minus set of terminals. The voltage (energy level of the crystal) can be measured by using a DMM connected to these terminals. These and some more aspects of the checker operation are depicted in the pictures below.
|Note that the meter output terminals are just leftover component leads suitable for alligator clips.|
|Here are the clip leads in place.|
|Here's the unit under test,|
|..and the frequency and voltage output..|
|..and, finally, the scope output.|