gents, this issue is a non-issue.
transmissions, while IN GEAR and idling are pre-loaded- meaning all the planetary and sun gears are pressing into position awaiting the release of the brakes. well sht..... this is gonna get winded, but it may help some of y'all:
if you can imagine a bucket of water and a person holding a paddle with intent to stir that water you have the general idea of fluid coupling. when the person starts stirring all the water starts flowing in the direction the person is stirring it.... the paddle represents the engines crankshaft.... the water is the transmission fluid in the torque converter, and the bucket is the torque converter housing.
imagine now there is a finned wheel in the bottom of the bucket with a sealed shaft that extends through the base of the bucket. when the input shaft (the paddle) is fired up by the engine (the person stirring) something has to give, or the engine won't be able to turn the paddle- meaning the engine (person) will stall (quit turning) or something will break.
imagine that doesn't happen, and the paddle turns freely in the bucket... in about one and a half turns the water in the bucket is starting to swirl in the direction intended- and the fins on the wheel/plate in the bottom of the bucket start to spin in tune with it. that is fluid coupling. the other side of that finned wheel/plate has a shaft on it which is the output shaft that feeds your input shaft which is what makes your transmission spin...
at rest the juice in the torque converter and transmissions are just that- at rest... still....
when the spinning begins, and fluid coupling happens (even though it is encountering resistance from the brakes being engaged) slack is removed from the spaces between the teeth on the sun an planetary gears (and other items of black magic present in an automatic transmission) throughout the transmission- and that which is represented as a lunge forward. that is all.
there are different types of stalls when discussing torque converters- the most common one is the speed the engine turns before the fluid coupling has the power to overcome sitting still- the point where it engages. I don't know what the OE stall speed is on these things, but I can say on the 09 superduty pushing the 5r110w transmission is varies depending on how much power torque management is scripted to deliver to the transmission- i'm thinking the stall speed is fairly low, like in the 1500~1800 range at the highest, which allows the transmission to engage quickly instead of suggesting the engine reach a higher RPM (where a 'built' for racing engine encounters the bottom of it's designed torque curve).... so.... when a person fires the truck up first thing in the morning it encounters a resting NOT 'preloaded' transmission and drive line (slack present in the gearing), and what does a cold mod engines PCM instruct it to do? a: deliver more fuel/run a higher RPM to get things going.... this is a higher rpm than you'd encounter on a warm start, and it's enough to 'preload' the transmission and almost enough to overcome the brake stall speed.
shorthand: slack in the gears, fluid coupling, higher initial idle speed on cold engine = lunge. not a problem.
