Because the output circuits of logic gates are a type of push-pull or ‘Totem Pole’ output with only one of its two output transistors conducting at any one time, when the output terminal is at logic 1 T4 is turned on the output terminal of Gate 1 will supply (or SOURCE) current via T4 and D1 to the input of Gate 2. This will cause gate 2 input to also be at its logic 1 state as shown in Fig. 3.3.5 (a).
When gate 1 output is at logic 0, T4 is turned off and T3 is turned on, and output current will now flow in the opposite direction, from the input of Gate 2 in Fig 3.3.5 (b) and via T3 collector and emitter to ground; this is called SINKING the current.
When a LS TTL gate output acts as a source, a maximum source current of -400µA is available to be drawn from the output terminal. Note that the minus sign used in this case signifies a current that is flowing FROM the gate output. When the output is sinking current, the LS TTL gate is able to sink 8mA. Notice the sink current has no minus sign as it flows into the output terminal.