Power of the Sniff
I think that this opinion is completely wrong. While you can get some good results from irrigation under pressure of the nasal passages, there is a downside. Very often there is a mucus plug in the sinus opening which is blocking the sinus. This can happen in either the case of allergy or in the case of a sinus infection.
When you irrigate the nose under pressure, you might dislodge the plug-- or you might drive it more into the sinus cavity. Now if you drive it more into the opening of the sinus you have made matters worse. Not only that, it is much more uncomfortable when you irrigate the nasal passage under pressure with a nasal saline solution compared to the gentle sniffing of a nasal saline solution or the gentle spray from a saline nasal spray solution.
Imagine a situation that is already uncomfortable. Then you take this higher pressure water hose and spray it up your nose! Wow, that feels good! The alternative is to take the nasal saline solution in the palm of the hand and, when assuming the correct position (see my blog) sniffing the solution from the hand.
When done properly, air is mixed in with the solution, and this causes a turbulence which gently irrigates the inside of the sinus cavities in a better way. The turbulence cleanses the inside of the sinus cavities in addition to the irrigation of the nasal saline solution.
So, when you consider what the two forms of irrigation do to your nose, what do you prefer? Personally, I would rather sniff the solution and therefor irrigate the the nasal passages and cleanse them at the same time, rather than exposing infected tissue to high pressure. To make the sniffing process work all the better, I have discovered a technique that I explain in my blog.