I like to look at it as there are two different types of tweak.
I refer to the first one as 'chassis tweak', where the chassis becomes twisted and you have to loosen either, or both, the top deck and/or the chassis mounting screws from the bulkheads.
I refer to the second one as 'spring tweak', where there are slight differences in spring height and/or length of the ball ends (where the lower collars settle onto them) between a front or rear pair of shocks.
Even with zero chassis and spring tweak, here are a few things that I've encountered myself that can throw off the readings on a tweak board:
* Uneven shock lengths
* Bent hinge pins (or even the screws for their suspension holders on an XRAY)
* Bent bulkheads
* Mis-aligned shock towers, where there is a little play between the mounting screws and the holes in the shock tower
* Major differences in static weight distribution at either end of the car
I've described how I correct both types of tweak in the Pre-race Alignment & Setup Tips section of the XRAY forum Tech Articles thread
. Currently I use the Hudy tweak station for step #9.