Sinking sensors allow current to flow into the sensor to the voltage common, while sourcing sensors allow current to flow out of the sensor from a positive source. For both of these methods the emphasis is on current flow, not voltage.
By using current flow, instead of voltage, many of the electrical noise problems are reduced.
When discussing sourcing and sinking we are referring to the output of the sensor acting like a switch. In fact the output of the sensor is normally a transistor that will act like a switch (with some voltage loss).