Interest rate and duration relationship

Hi everyone,

could someone please explain what this means “A portfolio manager seeking to increase the duration of the portfolio in anticipation of declining interest rates can long a bond futures”.

If interest rates decline, then bond values go up. I do not understand why the need to increase the duration because of declining rates?

Many thanks.

if rates decline in the future, you wanna increase your duration by now because duration approximates the change in portfolio value per 100 bp of interest rate changes.

so when the rates do decline, the high duration means your portfolio increases much higher in value compared to previously lower duration.