Pardon my ignorance but what and where is the OBD2?
OBDII is the interface standard all modern cars are required to use to communicate emissions status information to control stations with. There's a trapezoidal plug under the dashboard on the drivers side on any modern car, including the Volt, and like most of them, the Volt offers direct access to the CANBus on a couple of the pins (the CANBus is the car's internal version of ethernet.)
With a suitable device plugged into the ODBII port, you can therefore read every message flying between the various controllers on the car on that bus, including engine RPM and throttle settings and a whole bunch more (some cars, including the Gen 1 volt, and likely the Gen 2 Volt, have multiple networks, and you won't see the others from that one connection.)
Historically there have been dedicated devices for this, like the ScanGauge II (Don't use this one on a Volt, it slows the connection and causes errors,) or the DashDAQ. More recently, the trend has been towards simple Bluetooth or WiFi dongles that go into the plug, and Android or iOS software to interpret what the dongle sees.
Most of the BT dongles are fine, but we have had a few reports of problems. If you plug on in, and then get a check engine light and see a string of Uxxxx codes (you'll be able to get codes from any of these devices, too) then you've got a dongle that isn't compatible with the car's network. This can sometimes happen after it's been working fine for a while.
Here's how I used to have my DashDAQ set up, using entirely OBDII information:
As you can see, I have exact battery SoC referenced to when the engine comes one, and power flows from the battery, the engine, and both drive motors, along with RPMs for all three.