Mechanism of Source:

At the source, the food (sugars) is moved by active transport into the sieve tubes of the smallest veins. Due to the presence of sugar in sieve tubes, their solute concentration increases, and water enters them from the xylem via osmosis. This results in higher pressure in those tubes, which drives the solution towards the sink.

Mechanism of Sink:

At the sink end, the food is unloaded by active transport. Water also exits from the Sieve tubes. The exit of water decreases the pressure in Sieve tubes, which causes a mass flow from the higher pressure at the source to the now lowered pressure at the sink. In other words, the mass flow is caused by drops in pressure at the sink as the food and water molecules are removed.