Okay, you did help me understand a bit but I still need some clarification. So an external flash can operate in two different modes. On the hotshoe of the camera, and off. To operate with the external flash off the hotshoe, you can have it in slave mode. Slave mode from what I understand is that the flash can only go off if a master flash goes off first, to set it off, which would explain why it would be difficult to use in the outdoors due to sunlight. This whole time I thought transmitters/receivers and master/slave were the same thing sort of. I thought the t3i had a transmitter from the built in flash that would send a signal to an external flash, a slave, and then it would fire. In a sense, that is what happened but instead of being a transmitter, the built in flash was just the master and the external flash was reacting as a slave? I'm unsure if I'm being clear at all.
So what is commander mode on a t3i actually? Just master capability?
As well, from my understanding, a transmitter connects onto the hotshoe of the camera and then a receiver attaches to the external flash. Taking a guess on how it works, is it 'tricking' the external flash into thinking it is on the hotshoe of the camera? That way, it fires as if it were mounted normally, as opposed to being in slave mode, looking for a master flash (on camera flash) to fire?