On page 170 of VRM, GARP says that when key rates are defined in terms of par yields, one can immediately calculate the position necessary to hedge a portfolio once the exposure of the portfolio to the key rates are calculated.

I think I am struggling to see why all the changes in the par yields should add up to the yield-based DV01 (as the book mentions on p170). I understand that making a 1bp change in all the spot rates is equivalent to making a 1bp change in all the yields, but I am not sure how this links to par yields, yield-based DV01 and computing the necessary hedge positions.

Thanks in advance

Rohin