The Purpose Of A Mechanic
Before we can talk about showcasing a mechanic we need to know the purpose of it. Mechanics have a specific job, and that is to facilitate the resolution of a particular action that the game expects to come up frequently. If your game has a Mass Battle section, then the designer expected many GMs to be running large battles. If your game has a Fear mechanic, the GM wanted fear to be a feature of the stories told. You get the idea, right?
Now that you know that, what does the mechanic you are interested in facilitate? Is it a way to represent how terrifying monsters and certain situations can be and what that does to the heroes? A means of quickly representing the miles and miles the PCs will regularly travel? A way to determine the morale of underlings hired to do whatever job the PCs have?
Once you know what the mechanic does, you know what your session will need to work around.
But, No, Really, What Does It Do?
A whole session for a simple mechanic is a bit silly, no? But this isn't just a demonstration of a mechanic, it is a show case. Take some time, read through the section, and really think about how this mechanic could work. yes it has a job, but what does it represent in that job and how does it do its job?
Now a days you can often get some fun ideas by finding your RPGs online community and asking people for help. Lots of people are willing to give ideas and other thoughts on how to use mechanics. Read them, work with them, and do it.
Not All Mechanics Are Fit
Some mechanics are pretty simple and can't hold the weight of a whole session. Others are basic. A session built around the combat system is fine. Do it too much and your players wonder where the role play is. Do it too little and they get antsy to roll them bones. As such, combat - beyond the first session - may not be something you showcase so much as something you use on a regular basis.
On the other hand, something as simple as a Fear mechanic may be too small and narrow to really feature. In some games Fear is just a "make a will save or take penalties" and that isn't hard to work with but it also isn't going to hold a lot of attention. Contrast with systems that have very nuanced fear checks with multiple tiers and descending levels of sanity, and you've got something to really sink your teeth into.
As a rule, if you're having problems figuring out how to build the session around the mechanic you will probably need to look elsewhere. It doesn't mean the mechanic can't work in the session, just it won't be able to carry the whole job.
Execution Is Everything
Finally remember to prepare your execution. When working on a single mechanic as the focus for a game you need to do two things:
- Show and teach how the mechanic works and what it does
- Make it fun
Making it fun is the hard part. This can mean really playing up the descriptions, going nuts with the encounters, or any number of things depending on your players.
The fun is key though. You need it to be fun. Good luck!