If the soil is near saturation, the infiltration rate will be slow. The antecedent moisture changes during a given precipitation event so that the infiltration rate decreases as it rains. If the rate of precipitation is greater than the rate of infiltration, then there will be overland flow. Matric force and gravity combine. Baseflow is the water in the saturated zone, below the water table, moving down and to the side.
Gravity is the force.
A hydrograph represents the response of a stream to precipitation. Reasons for a stream being flashy include infiltration rate, drainage density, and basin shape. It can and does, but in more heavily vegetated areas there are flashy hydrographs with no observed overland flow. Throughflow can produce a flashy hydrograph by leading to saturated soil surface conditions near the stream. This type of responses, which changes through the course of a precipitation event, is called a variable source area response.