We are constantly reminding our students that in a real situation they will experience an adrenal dump (as well as the endorphin release that comes along afterwards), and that that adrenal dump will have a number of effects. In short, we usually say that it will make you "stronger, faster, feel less pain". The point we also make to them is that the opponent will also have an adrenal dump... making them stronger, faster, feel less pain.
There are other effects, though, which we also go through, such as tunnel-vision (leaving you open to wide strikes and other attackers), a loss of higher brain functions (including the ability to form sentences), a loss of fine motor skills (such as intricate locks and holds, and fine targeting). In regard to the OP, another aspect is that it triggers your fine-twitch fibres, in preparation for whatever action is needed (specifically flight or fight). This doesn't mean that you're scared (as can be misinterpreted), just that you're getting ready for movement.
When it comes to the fight/flight/freeze responce (as well as posture and submit), there are a few factors to determine what will happen. The first thing is that you get an adrenal dump. Okay, we're there already. The next major factor is often distance. You (or the opponent) will be more likely to fight in a close distance (within about 6 feet), and more disposed towards flight from a greater distance (over 10 feet). Between those can be a grey area, and will come down to previous experience more than anything else. Freeze is what happens when you are within the "fight" distance, but have no prior experience or confidence in your fighting ability (in other words, there is no strong "fight" responce loaded into your unconscious responces, or you have a value system that does not include fighting ability, either you have unconscious programming that you shouldn't fight, or you don't truly believe that your training will work, etc). When it comes to posture or submit, they are animalistic responces (hard-wired into the subconscious on a genetic level), and are simple survival actions. Both simply seek to avoid danger and violence, the posturing to appear too dangerous to attack, and the submitting by appearing to not be a threat.
As a result, I feel that it is vital to not only train with adrenal-stress training, but also to understand the why's and wherefore's of such training. The majority of your training (if you are being realistic) should always have at least some component of adrenaline training involved. But you can't just train with adrenaline and hope that'll be enough. You need to train with the adrenaline, and then focus on pushing past the limitations as much as possible. For example to overcome the effect of having your vision tunnel, get yourself adrenalised, then focus on your peripheral vision. This way you will gradually limit how much your vision tunnels.
