See M. El-Erian last articles about credit quality erosion.
US market has near 5 trillion of A grade and BBB bonds.
Economic shock may cause downgrade of these bonds to junk levels which will launch sell-off of these 5 trillion papers.
So we take Ishares quality bonds ETF which includes only AAA bonds (QLTA) and deduct from it performance of Ishares ETF which buy...