A common example of a bull bond is the principal only (PO) strip mortgage-backed security. Whereas most bonds will increase in value in a declining rate market, mortgage-backed securities perform especially well. POs are mortgage securities created by separating principal payments from interest payments collected in a pool of mortgage securities. The principal payments are then combined to form a mortgage pool. PO mortgage securities do well in a falling rate market because mortgage holders refinance their loans at lower interest rates. Investors are repaid their original investment more quickly, increasing the rate of return for the mortgage-backed security.
Investment dictionary. Academic. 2012.