Please bear with me, I need to set the context first. If it's TLDR for you, just skip down to the link and take a look.
When I first joined this site, the general tone on helicopters was that they are "just the same as fixed wings except that they rotate their wings and, except when hovering, have only minor differences in flight".
Thanks to the efforts of people who actually fly them or have other detailed knowledge, we've completely changed this and I believe the site is better for it. Folks like rbp, mins, aeroalias and korinstormast (thank you guys) have added value and helped a lot of people to better understand rotor craft.
I am the kind of person who when I see something wrong somewhere I can influence, aim for one of two outcomes. Either I change my position because someone explains why I'm wrong or, I correct the mistake. My preference is the former. I believe that this is especially important here since our primary goals are to help the most people possible and provide the best Google hit. Letting bad answers live, without resolving disputes on them, harms that goal.
Finally, emotion. I hope that the old hands around here view me as helpful, respectful, good humoured and accepting of the wishes of the community. I strive to do this by keeping emotion well out of it. I learned the lessons of emotions on the internet a long time ago. I occasionally slip, especially when I've been enjoying Scotland's finest export when I get a little bit cheeky, but my personal goals when I come here are as above, and in that order.
So, the point of all of this.
My simple approach. Centrifugal force is fictitious. It exists only as a result of calculations required to explain real effects observed when looking from a rotating frame of reference to some other. Please, do not debate this further here. If you doubt the truth of this, visit Physics.SE or other canonical reference to satisfy yourself before proceeding. Centrifugal force can have precisely zero effects in the real-world. Rather, it arises as a result of observing and explaining the effects of other forces.
"Stiffens". The only way to stiffen an object is to change it's physical characteristics. Aside from some relatively minor side-effects arising from the change in shape of the blades when rotating, their physical characteristics do not change.
Unfortunately, Ray Prouty used a similar phrase in one the helicopter worlds most respected works which is being used to justify the answer. Sadly, we can no longer ask Ray so I am guessing when I say that I suspect "stiffen" was a poor choice of word to mean "resists and counters the bending force" and "centrifugal force" in recognition that the majority of people do not understand (or care about) the correct explanation. Perhaps a better phrase would have been that "the blades resist bending forces as the vertical component of the centripetal resultant force acting towards the hub and lift increasing as the blade accelerates, causes them to rise".
Had this phrase, or similar, been used in this answer, I would have accepted it except for my final objection to this answer which is that it does not answer the question posted.
To close, if I am wrong, then I will enjoy my slice of humble pie, walk away happy having learned something and make any apologies I need to. As it stands, no-one has offered any help as to why I am wrong so my objections stand.
I've already flagged as "does not answer the question" but I can't see the outcome and there is nothing I am aware of to let me see if the community has already decided to accept the answer or that it simply did not make it to the review queue.
Forty plus years of education and experience in technical and engineering disciplines yells at me the that this is wrong but my physics education is only just past the point of understanding the centrifugal force fallacy since my degrees do not major in physics.
Please help me to resolve this dispute.