In two ways. First, the physical presence of the starch - a solid - lowers the proportion of the liquid in the sauce and therefore increases the sauce's viscosity. The second way is more complex and more significant. Each of the minute starch granules traps water molecules and, in the process, reduces the proportion of free-flowing water in the sauce. When the starch is heated, the molecular structure of each granule stretches, thereby increasing water retention.
** Asian Recipes