A transistor has 2 junctions. They are physically close to each other. They are so close that if you inject a current forward bias into the base emitter junction, holes or electrons ( depending on PNP or NPN ) get pulled by the reverse biased base to collector, into the depletion region of the collector to base. Once in that region, they can't go back, against the voltage, and become collector current.
Surprisingly, if you turned the transistor upside down and wired the emitter to the collector circuit and the collector to the emitter circuit, it would still work the same but with reduced current gain ( as long as the collector circuit voltage was less than about 6.8V ). If the voltage was too high, the emitter to base would zener.
In order to analyze the typical TTL input you'd have to understand how this happen. When the input voltage is high, of a TTL, the emitter is working like a collector and the collector is biasing the next transistor on, as though it were an emitter. In a regular TTL load there would be about 1.6ma flowing forward bias from the input transistors base, forward biased through the collector junction. As I said, when the biasing of the transistor is reversed, there is some gain making the emitter look like a collector, supplying 40 ua on top of the 1.6ma into the following transistors base lead, turning it on.
I hope that helps a little.
To get your terms right, Forward bias is current flowing, reverse bias is less current flowing. When you turn a transistor on, the collector current is flowing through what otherwise would have been a reversed biased junction except for the stray current carriers from the base, cause by the forward biased base to emitter. You can swap emitter and base leads and still have transistor action, just not as much gain.
I should mention, the emitter base junction is highly doped, making it zener when reverse biased at about 6.8V, for a typical transistor.
Dwight