Simplified one-line diagram showing the effect of tapping a Quadrature booster (shaded grey). 
Both images show a notional 100 MW generator supplying a 100 MW load through a pair of lossless transmission lines, one of which features a quadrature booster with a tap range of 1-19.
In the left-hand image, the quadrature booster is at its centre-tap position and has a phase angle of 0°. It thus does not affect the power flow through its circuit and both lines are equally loaded at 50 MW. The right-hand image shows the same network with the quadrature booster tapped so to buck the power flow. The resulting negative phase angle has transferred 23 MW of loading onto the parallel circuit.

(The values used here are arbitrary; the actual shift in load would depend upon the design parameters of the quadrature booster and of the transmission lines.)
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BillC