Dear Diary

The value of a  personal written debrief (PWD) is well established in research. PWDs reinforce positive learning opportunities, reduce stress, and can improve performance of physical and mental tasks. The key to these benefits is the way the brain receives, processes, and stores information. To be most effective, PWD requires 15-30 minutes of focus with minimal distractions. This usually means that doing PWD on duty is less than optimal, but possible.
The basic tools, other than time, are a notebook and good pen. Yes, PWD is writing stuff down. In education it's called reflection, in therapy it's called journaling, and to teenagers with a denim covered book inked with doodles and containing tear-stained secrets it's called a diary. I'm painting it Spec Ops camo and putting it in a black nylon tactical combat holster so cops won't worry that reflecting on their day is too "touchy-feely".
PWD works because it helps wire the brain by creating and establishing a memory file that will be used in future decision making. We know that bad experiences and traumatic experiences are imbedded more deeply than good experiences. The brain is much more invested in avoiding extinction than it is in making you feel happy.
What that means to you is that if you do something good or have a good experience, the brain notices it less than a bad or threatening experience.  A bad experience is more likely to be forever recorded (even if inaccurately) for use in future decision making. Think of the old management rule that it takes a bundle of positive affirmations to overcome one morale-killing criticism. PWD helps cement those positive attributes of your encounters and, most importantly, can help you correct any errors or less than stellar performance on a given call. If you've screwed up, using reflection to replay and repair the scenario in your head can help minimize hesitation or avoidance in the next encounter and create new habits and responses.
How does PWD change behavior? A study showed that performance of a group of non-piano players that practiced a piece on a keyboard, and a group that practiced only mentally by visualizing practicing on a keyboard, had similar performance outcomes! In other words, mental rehearsal has real value.  
Another reason that recalling and replaying an event is helpful for memory is that the act of writing uses the sense of touch, watching the words flow onto paper uses the sense of sight, and both ignite a replay of the event in your mind's eye. Multiple sensory engagement plus intentionality means increased retention. Asking yourself what you learned today might get the same response that your kids give when you ask that question at the end of the day: "nothin!". It takes intentional reflection to sift the value out of your day's experiences.
Another value of PWD is gaining emotional control over the events of the day. There are many days when the last thing you want to do is to think about your shift. You want to just relax and forget it. Perhaps on an awareness level you can do that, but there is often a part of your brain still churning things over and creating tension and anxiety at a sub-awareness level. Writing things out provides a way to process and contain those events, resolving them to some degree so that you can get them "off your mind" by getting them on the page.
Finally, because much of our routine daily activity is on autopilot, we assume that we're operating optimally if we have a normal day. The opportunity for self-improvement in our job performance, relationships, eating habits, fitness routines and other areas of life comes from a self-awareness that can only come from intentional reflection.
We like to talk about the warrior spirit and a survival mindset. On a shallow level, many interpret that kind of character as one in which we must ignore and deny our imperfections and pretend to be tough.  A more honest and mature definition is a confidence that comes from mental fitness and self-awareness. PWD is a great work out for top performers.  Just don't tell anybody you're keeping a diary. They will snoop.