Constriction of the Afferent and Efferent Arterioles
Normally the afferent arteriole is of larger diameter than the efferent. This means there is high resistance as the blood is forced from a wider vessel to a narrower one and this promotes filtration. If the arterial blood pressure remains constant then contracting either vessel reduces blood flow as it increases resistance. However contracting either has opposite effects on the filtration pressure. If you contract the afferent arteriole there will be less of a pressure difference between the afferent and efferent arteriole so there will be reduced filtration pressure. However if you constrict the efferent arteriole you are increasing the pressure difference between the two and filtration pressure increase.
Overall the constriction of the afferent arteriole decreases both blood flow and filtration pressure where as constricting the efferent arteriole decreases blood flow but increases filtration pressure. (Both of these statements are assuming a constant blood pressure). The fact that both can be altered allows independent regulation of both GFR and blood flow.