Ever since 1987, banks have been willing to cut rates when markets have wobbled. Driving down bond yields has been an explicit aim of QE (and pushing up stockmarkets an implicit one). Currencies move in anticipation of interest rate divergence (hence the strength of the dollar this year) and a stronger/weaker currency has a tightening/easing effect on economic conditions.
A related question is what exactly is meant by tightening effect on economic conditions. This term is meant loosely in financial media and I do not know exactly what it means.