This is pretty clever – and it seems like it would work –
One of the trade periodical suggests creating a barrier around the stain as it tries to spread with water or a peroxide carpet cleaner.
The theory being some variant of entrophy – that the the staining fluid will migrate to dry carpet fibers. By wetting the surrounding fibers the migration is stopped / dammed up.
Once the barrier is created and the stain stops spreading and you only have to blot up / extract a smaller area.
Similarly if you are around when someone spills coffee or soda into the carpet, the first thing you should do is get some dry fibers (a towel, rag, or paper towel), place the dry fabric on top of the stain, and apply pressure (stand on it with your shoe) so that the dry fibers come in contact with the wet and blot up the liquid.
Repeat with unstained dry rags etc until you are no longer picking up any moisture. I’ve come close to firing two operations guys for NOT doing this when they spilled in my office on the assumption that they could come back in the evening with a piece of equipment – but minimizing the amount of fluid in the carpet and minimizing it’s spread make a huge difference in how clean you finally get the carpet when you do bring equipment in.