hmmmmmmm maybe, but I've hung out at skate/snow shops for a large chunk of my life and have not heard flexation. not saying that it isn't something that isn't used in your area, but using the fact that the word "blog" wasn't in the oxford dictionary doesn't stand up very well. I will support this by pointing out that the word blog WAS MADE UP by a company to name a product, and people who were in on this new internet phenomena attached meaning the words new meaning. Its not like they just made up a new word. Flexation is a modification of the word flex, and the suffix ation that means something is in action, so flexation could mean the act of flexing, however I do not see a specific use for this word where another more formal form of the word flex could be used. Personally I think all of you boot fitters just made up this word because you wanted to make yourself appear to know your shit when your making a sales pitch about a certain boot. If you find me a legit internet boot description that says this boot has soft flexation, or something similar instead of it saying this boot has a soft flex I will accept the word flexation.
This is my point they put tons of other modified words in the dictionary, like articulation. there just it isn't linguistically correct to put the suffix ation behind flex.