It has been discussed in detail in the forums many times however its probably mixed in with a bunch of other threads. In a nutshell, keeping the power curve more consistent by holding boost between shifts and having the fastest shifts possible keeps the preload on the drivetrain gears engaged more. When granny shifting you lose all boost (which hurts performance between shifts) and takes slightly longer to shift. During this time the preload on the drivetrain gears will drop rather quickly. Once you re-engage the the clutch and mash the gas, the slack in the drivetrain will now abruptly slap closed and that is like hitting the head of two hammers together very hard. Thats as best as I can explain it since its not an area I consider myself to be an expert lol.

Its similar to the rule about launching at the 1/4 mile track. If you were to try and roll forward to trip the pre-stage lights and you went too far, some people pop it in reverse and roll back until they trip the lights correctly and stop right there. This is a horrible thing to do. Since you went in reverse, you have the gears preloaded against the back side of the gear teeth. When you launch the car the gears will quickly slam into the FRONT of the teeth, closing the gear lash gap abruptly with a great deal of force. This has been responsible for MANY broken drivetrain parts at the starting line.

If you ever drive too far past the pre-stage area, roll back a few feet past the pre-stage lights and then re-approach the lights again in 1st gear to pre-load the gear lash on the front of the gears teeth. Now when you launch it just 'moves' instead of a violent SLAM, then it begins to move.

Here is an example. Have someone hold their fist out firmly with a straight arm and have them stand 1 foot in front of you. Tell them to fall forward will all of their weight so their fist makes contact with your chest. When they make contact with you, its gonna hurt during impact....then its gonna push you backward a little.

Now have them do the same thing but this time have them lean forward just until their fist is making contact with your chest. Now have them lean all of their weight forward. This time the force from their weigh will still move your body backwards, only without the violent impact just before you get pushed back.