What made you want to look up low-pressure? Please tell us where you read or heard it (including the quote, if possible).
While spaying is traditionally recommended for kitties between 4 and 6 months of age, spaying as young as 8 weeks old is considered safe, according to Cat Channel. Many animal shelters actually spay kitties prior to adopting them out at this age.
A healthy kitten can be spayed or neutered once it's eight weeks of age and two pounds in weight. Sadly, though, a lot of vets are behind the times and still wait till six months. Considering kittens can reach sexual maturity as early as FOUR MONTHS this is a bit ridiculous.