This method assumes that you’re using a minimal amount of processing–an amp and cab on a single row, plus a small number of other effects that can fit–so that you can switch between rows.
Toggling all of those blocks on and off can help to visualize what is in use, but in terms of sound it’s more important to toggle the output of each signal chain so there’s no noise or bleed going through to the output. On each row you can use a Gain block, with the gain set all the way down, and turn it on when you want to turn the row off.
It gets more complicated when you start adding more effects blocks than can fit on a single row, or when you need to split signals to FOH and a separate power amp and cab setup, or when you want to use the same block for multiple sounds but want to change parameters within it, etc. This is where Scenes become far more useful.
I’m not convinced that the amp-and-cab-on-a-single-row method is the best optimization for the QC’s grid. I just posted about this in a separate thread, give it a look.
EDIT: Yes, @rangelmarcelo, rows 1 & 2 are handled together with half of the QC’s processing power, and 3 & 4 with the other half. If you’re using really CPU-heavy effects everywhere, it’s a good idea to split them up between Rows 1 & 3 instead of 1 & 2. Also, the global EQ is applied with the same processors as rows 1 & 2. If you bog down 1 & 2 too much, the global EQ will not be available to use.