Fixed Income : Classical single immunization

In general, for an upward-sloping yield curve, the immunization target rate of return will be less than the yield to maturity because of the lower reinvestment return. Conversely, a negative or downward-sloping yield curve will result in an immunization target rate of return greater than the yield to maturity because of the higher reinvestment return.

Source : Reading 20, 4.1.1.2

I cant find the rational. Any explanation is highly appreciated.

please use the search function … and this has been discussed ad nauseum on the forum.

From http://www.analystforum.com/forums/cfa-forums/cfa-level-iii-forum/91349096

quoting S2000