ah the hobbs switch... modernly its used as a switch to turn things off when $#!t hits the fan, i.e.: turn off the nitrous when the fuel pressure drops, turn off the nitrous when the turbo is spooled, turn off the nitrous when the bottle pressure drops... infact its used to to turn off nitrous alot now that i think about it....
however...
using a hobbs switch on a turbo motor can do some interesing power enhancing things too. lets say i had a stock turbo drivetrain but wanted to increase the boost past the realm of capabilities for the stock fuel system but didnt want to lose my stock style driveability or my cruising comforts while in vacuum. well, i could tap the manifold and install a hobbs switch rigged to an extra injector to come on at say, 10 psi. and "tune" the combination by playing with injector size and number of injectors, for example.or staging them with multiple hobbs switches so one injector comes in at 7 psi, another at 12, another at 17, and so on.
once upon a time before the wisespread use of in car lambda (wide band and narrow band afr gauges) we would just make sure the turbo cars stayed huffing black diesel type smoke throughout the powerband at WOT so we knew we were running rich and safe.
msd used to sell a "engine knock alert system"
http://www.summitracing.com/parts/msd-8964
that we could use that has a simple bar graph type (green bars, yellow bars, red bars!) to tell you when you have a dangerous amount of knock. this was a grea ignition tuning tool back in the days because it allowed you to move your timing to the threshold of detonation and have a good idea what was going on inside your combustion chamber.
when combined with another indispensable msd product, the BTM or boost timing managment system
http://www.summitracing.com/parts/msd-6462/overview/
you could set up your initial timing to really wake the dead "off boost" and come into boost quicker with less lag, knowing that msd BTM would pull one, two or three degrees of timing for every pound of boost it detected (up to a total of 15* retardation) just by twisting that fancy little knob that it comes with.
let me tell you seeing a turbo six cylider eighties super car with digital dash instruments and crazy msd gizmos all over the center console and the "oh my god, why does it have twelve fuel injectors if its only six cylinders?!" made quite an impression on a younger version of me once upon a time.
im gonna glaze over the fact that everyone with turbo cars in south florida in one night shut down the airport because somehow all of the isuzu NPR trucks used for everything from baggage and cargo moving to waste management had been relieved of their T04B/E/and S turbos and of their very efficient intercoolers that fit perfect in the front of a z car. i dont condone stealing but i was young, broke, and naive and wasnt about to look a cheap deal on some parts in the mouth. nowadays theres better and cheaper stuff available new than ever so its not nearly the epidemic it once was down here.
ah but i digress...