I have been studying and testing what the User Guide says about the TEST button. By good chance, a question was asked this afternoon in the Strobist group that yeilded more info.
Here is my amalgam so far. Hopefully, someone can test the light meter connection.
Testing the lighting setup
The setup can be tested to ensure that all devices are powered up and that the communication is working. In addition, actual lighting outputs can be metered if manual levels are being used.
• The [Test] button can awaken the flashes and test whether they are being triggered.
• The test works through both hot-shoe and PC-sync cable.
• The flash will wake if necessary. If it does not, manually awaken it.
• Use the PC-sync port to connect a flash which does not have the awakening function.
• All transceivers (transmitter or receiver mode) on the same channel as the test unit will be accessed.
• Those flashes included in the “Testing Group” will be fired.
• An on-top flash will also fire if Group A is included in the test groups.
• All the indicators go out when flash fires.
• Testing group is different than the Firing group, which controls ratios and levels.
Testing all flashes
• Press and release [Test]. The default group is All (A+B+C).
• The group indicators will light, the flashes will awaken and fire almost immediately,
Testing selected flashes
• To select a testing group, hold down [Test] and repeatedly press [GP SET] to cycle through the seven testing groups.
• When the desired testing group is shown, release [Test]. The flashes in the selected group will fire a test flash.
Using a Light Meter
• Use Manual levels (or Multi), set either by Remote control menus or on-flash settings.
• The test fire will be at the level as it is displayed on the LCD of the flash.
• The test button will not change the setting of the flash.
• If Remote control is being used, press half-shutter to ensure that settings are propagated through to the flashes.
• E-TTL will produce a pre-flash, which may confuse the light meter.
If the transmitter is in mixed control mode, set the flash manually and trigger with any YN-622c on the same channel.