So how does this work, does it sense an 'imbalance' between the '2' hot legs??
It doesn't. Imbalance may not be the best word to use when comparing 2-wire (1p) and 3-wire (2p) GFCI breakers. You could have 2, 3, or 20 wires pass through the CT donut inside the GFCI device.
As long as all of the current going to the load on the ungrounded conductor(s) is matched by the current returning on the grounded conductor(s), no voltage is developed in the sensing coil on the CT.
If any current in an ungrounded conductor returns to the source's grounded conductor (via the GES) through other than the circuit's grounded conductor, it may be passing through a person's body.
That's why a GFCI device needs a grounded system to detect accidental hot-to-earth contact. If the system is floating, the contact won't cause a current high enough to be dangerous (theoretically).