Every time you buy, you split $$ into a basket of coupon mix. And as yield goes up, you move up your coupon range.
For example, purely as an illustration
First buy when yield is 4.5%: 1/3 fund to buy 4.25, 4.5, 4.625
Second buy when yield moves up to 4.7%: 1/3 fund to buy 4.5, 4.625, and 4.75
Third buy when yield moves up to 5%: 1/3 fund to buy 4.625, 4.75% and 5%
