ALS how it works to us commoners.. I'm sure this is a vast oversimplification.. but here goes
When you lift off the throttle the ignition is effectively switched off (advanced massively so it can't create any cylinder pressure), as well as this I think the injectors get opened up to full duty cycle, now combine that with the Idle Air Control (what is normally used to allow a little air into the engine when the throttle butterfly is closed) is opened up.
So now you have raw fuel getting spat straight out the exhaust ports, no ignition, and some air getting let into the engine from the idle control. Add one hot exhaust manifold and turbo.. kabooom.. the air/fuel mixture lights up ... and continues popping and banging as uncontrolled combustion occurs. The energy from the gas expansion, as it's used to push the piston down normally pushes past the turbine keeping it spinning.
Now normally this really isn't that brutal because the idle air control can't supply enough air to make a truly effective bang (read big nasty WRC style anti-lag) so what they do is crack open the butterfly slightly to let more air in. Thus the engine never experiences vacumn.. great for anti-lag, sucks for brake boosters and clutch boosters (as they need vacumn) so what the rally guys do is attach some little vacumn generator which generates vacumn for the things that need it. This is the truly effective anti-lag system.. because clutch in on the start line at 4000 rpm or so you can generate quite a bit of positive boost.
The downside is that you have all these uncontrolled combustions which shock the turbine blades and rattle everything in the turbo/manifold area. Not cool.
(I think)
But then again with my Launch Control system I can generate almost .5bar boost at 5500rpm on the start line and if I had flat changing I wouldn't drop much boost on upshifts.. doesn't help downshifts and braking into corners.. thats where anti-lag is the absolute dogs nads. I'm taking some vids of the launch control etc this weekend.. infact i've got a couple already just can't figure out how to blank my plate out. With the TCS/LC system I get some of those big pops and bangs, because when TCS cuts in it effectively halves injector pulses as needed creating mixtures that won't lite up/produce any power... they come out the exhaust ports and give nice little crackles.. if you time it right in the 1st - 2nd shift or 2nd -3rd you get the nice woooompphss...
Rowan