Lean burn is the biggest reason why the manual transmission Insight was rated at 70mpg highway, and the automatic (which can't do lean burn) at 56mpg. When the engine is fully warmed up and lightly loaded, it can run at very lean fuel:air ratios, improving engine efficiency at the expense of power. If you're in lean burn and you press the throttle pedal too far, you'll feel a lurch (or not), the fuel economy gauge will plummet, and the car will go faster.
To get in to lean burn in 5th gear, hold the instantaneous mpg gauge around 90. Hold your foot steady, and it should shoot up over 120mpg. Then, unless you want to go 40mph, *gently* increase the throttle until the impg gauge comes back down to 80ish. 75mpg is the limit of lean burn in 5th gear. Below this, you can not lean burn, and your engine is operating at suboptimal efficiency. Spend as much time as possible in LB on a long trip, and your mpg will be much better.
If you do target driving, and your target is above 75mpg, AND you re-enter lean burn every time the mpg falls below 70, you win.
MIMA Honda Insight Modified Integrated Motor Assist - MIMA Insight System
~600 solder joints? Looks super-simple to me. *shrug*