Like many features on many homebuilts, the assignment of the stick buttons reflects the builder's eccentricities. Mine are affected not only by my experience flying rented airplanes for 20+ years, but also 12+ years of building and flying UAVs.
For example, the blue pinky button is pressed whenever I experience a "What the hell is it doing now?" moment. If the trim runs away, this normally-closed button kills the power to both trim servos. If the autopilot does something stupid (or, more likely, if I set it up to do something stupid), this kills power to it...completely. Once I get control of the airplane back, I can take the time to figure out what went wrong and fix it. This is direct result of many an "Oh, shucks!" in a UAV ground station. The blue button on both sticks is wired in series, so they both have to be there for the trim to work. If I ever want to make the right stick removable, I'll have to build a jumper to supply power to the trim through the left stick.
The green engine start button is on the knuckle side of the stick (i.e. outboard.) It's a bit unusual, I'll grant you. However, this comes from many frantic engine starts in a fuel-injected Citabria 7KCAB. I often felt like I needed an extra hand or two. One holds the stick back, one presses the starter button, another shoves the mixture in and pulls the throttle back. This little button allows a normal two-handed human to do it. The two sticks are wire-or'd together so that either stick can start the engine. And lest I be mistaken for a liberal arts major by keeping it simple, the green buttons are powered by both the main and essential busses through diodes.
Boy, I bet this will start a conversation on the forums!
The other assignments are pretty canonical: push-to-talk, CWS, trim, and com radio frequency swapping. I will of course label them in a Walter Mitty fashion: Phoenix/Sparrow/Sidewinder/guns, weapons release, nuclear consent, chaff/flares, etc.
The inside of the sticks is a bit daunting. It's hard to put it back together, but after a couple of times you get the hang of it. A word to the wise, though: it's easy to get a wire caught between two hard pieces. I had a vexing short to ground that resulted in a blown fuse in the trim circuit every time the stick touched the (grounded) airframe. It turns out that the wire the supplies power to the trim actuators (through the blue buttons) got squeezed. Its insulation was damaged, and it was shorting power to ground.
Like I said...lots of wires. That big blue wire is a bitch to feed through the stick, too. I terminated both sticks in a DB-25 connector. I know lots of builders install terminal strips galore under their floorboards, but I decided against that for a bunch of reasons: I don't want exposed conductors, I don't want more connections than I already have, and I expect switch function reassignment to be a rare event...it's OK if it's hard to do.
No reason for this photo to be here...but this is where iPhoto put it! It's official...this snap bushing is full. The left wing root has wires for: nav antenna, taxi light, pitot heat, nav light, strobe, and the status LEDs for the Gretz heated pitot. And a separate ground for each, because I'm an anal-retentive EE. Plus of course the the Tygon tube for the pitot pressure. I guess this means that the roll servo will be in the right wing...there's no more room for wires in the left wing!
OK, back to the Infinity Aerospace sticks. This is the bolt to retain the right hand stick. Yeah, friction would probably suffice...until it doesn't. There are enough stories floating around of crashes due to the stick coming lose that I figure a bolt is worthwhile. The thick cable won't allow the bolt to be drilled on a tube diameter, so I had to drill it slightly off-center...after I filed a bit of a flat on the tube so that the drill bit wouldn't chatter off. So my right stick is removable...if you remove a bolt and connect a jumper under the floorboards.
After a lot of work, the heinous bundles of wire are laced together and Adel clamped away from the stick linkage. The connectors are tie wrapped to the floor or ribs so that they won't flop around. I don't have a lot of faith in those self-adhesive cable anchors, so I only use them in places where they won't kill me if they come lose. If it really matters, I use an Adel clamp. Needless to say, I had to remove the control column, which pissed me off no end because it's such a nut-and-bolt-fest to get it back in. I borrowed some washer- and nut-holding tools from Dave Abrahamson, though, and that made it less onerous than it was the first time.
Those little black boxes that you see screwed to the ribs are the relays for the elevator and pitch trim actuators. They're made by Ray Allen Company. Their function is to let the stick trim buttons, which are SPST closures, control the trim actuators which have just two motor wires coming out of them (they also have a bunch of position-reporting wires). Those two wires get voltage in one direction or the other, and the function of these relays is to provide that switching.