DVDs hold a given amount of data (4.35 gigabytes user space give or take for single layer disc).
Wii dumping methods dump this entire space.
Games however need not take the entire disc (most wii games hover around 2 gigabytes).
In normal discs the unused space in between the data (or at the end of the disc) is not used or is simply long repeating strings.
In most wii games this junk data is not long repeating strings but long sections of essentially random data. The wii could not care less about the junk data between files (as evidenced by the few games that do not have "random" data as this junk data).
Random data does not compress so instead of including it scrubbed isos replace this data with long strings of 00's*. This newly tweaked disc will compress very nicely and allow easy storage, easier transfer via slow networks (the internet for instance) and now we have things like the USB loader easier storage on the USB/SD storage device (if I not mistaken the junk data is "lost" but it is "reconstructed" on the fly as there is little more predictable than 00's). As some games have this and the wii has little in the way of checking this is considered very safe to do, as we also own the wii's internals any "bans" or problems could be circumvented relatively easily.
*while the basic idea behind wii scrubbing is this the original wii scrubber application has extra features that can make things even smaller at the cost of needing the disc to be fakesigned (not a problem for those with a well hacked wii but potentially a problem for those just running a modchip and nothing else who decide to update menus or otherwise render themselves unable to run fakesigned code, if you are running a USB loader, a disc loader or a have a properly hacked wii then you have no worries about this). Such things include removing the update partition, shifting the data to the front of the disc and more elaborate things like multi game isos.
Of secondary interest here is the "leave the headers intact", while I said detection is hard there is the outside chance of a read into this scrubbed region (nothing has been said or done to give any hint of such a thing and given that we own the wii.....), by leaving the headers intact the wii can be duped even then but it comes at the cost of losing some compression (about 200 megs over the course of a regular disc if memory serves) note leaving the headers or not does not require fakesigning.
Other note basic scrubbing still needs the key to able to tell game data from junk data.
