The reading mentions about this equality on Page 300 –

“When a bond portfolio is fully hedged, its value is immunized with respect to changes in yields, stated as

ΔP = (Ns)(ΔS), where ΔP is the change in the value of the bond portfolio and ΔS is the change in the value of the swap for a given change in interest rates and Ns is the notional”

Where does this equality come from? ΔP = (Ns)(ΔS)?