Looking at the instruction manual for the listed unit where they mention mounting: Mount flat or on edge with arrow pointing forward. Mount module as level as possible +/- 20 Degrees. I think we can pretty much figure they use an accelerometer to detect deceleration. I liked crutschow's circuit. If I went with a uC I would consider a simple 8 pin flavor maybe like a PICAXE 08M2. Also with the originally linked unit they mention Sensitivity and how to adjust for Sensitivity.
4. Adjusting Sensitivity
When calibrated, the module sensitivity is set to
an ‘average’ level based on a number of bikes
tested from large cruisers to sport bikes. Most
customers find they want to adjust the
sensitivity based on their bike and/or riding
style. To adjust sensitivities adjust the angle the
module is mounted
1. To make module MORE sensitive change the
angle so that the arrow points more DOWN
towards the ground.
2. To make module LESS sensitive change the
angle so that the arrow points more UP away
from the ground.
Being a retro type dinosaur I have a 1992 Harley Davidson Electra Glide Ultra Classic. My bike weighs in at around 750 to 800 Lbs. When approaching a stop I generally downshift before I use any braking but that is not always the case. Also, originally my bike came factory with an 1157 tail / brake light bulb plus additional brake lighting on the tour package. The standard 1157 bulb on bright brake draws about 2 Amps. Depending on configuration a bike could draw 4 to 8 amps when braking. I replaced my lights with LED (keeping the original parts) which required adding a few load resistors on the turn signals as the controller looks for incandescent loads.
Anyway one concern here is deceleration on any bike will depend on the bike and the rider and believe me stopping a few hundred pounds of dirt bike or small light road bike is not the same as stopping or decelerating a large road bike like the 900 pound BMWs, Harleys or similar. A guy riding a bike wants high visibility. Just about every time a motorcycle rider is hit by another vehicle the other driver claims they never saw the bike. When braking a biker is always looking in his mirrors making sure the guy behind them has noticed them stopped or stopping and looking for an out just in case. Bikes want and need bright attention getting brake light systems but also you don't want a brake light for every time you moderately decelerate. When I brake I have a system which does three bright quick flashes followed by a steady on as long as I am using my front or rear brake which I am good with. Would I buy another system to brake light on deceleration? I doubt it but obviously there is some demand.
All in all there are a good number of things to consider when looking to trigger off deceleration. Engine RPM really is not a factor as when accelerating I roll off throttle to up shift. My tachometer looks like an intermittent yo yo getting on an interstate.
Ron