I had the same experience recently. Took my Niner RLT cross bike on some singletrack trails that I regularly ride my trail bike on. Set all kinds of PR's on the flatter and climbing sections. It was hella fun too! Just picked up some Heist 24 wheels to run 40c tires on, specifically when my ride involves singletrack, which it definitely will now. I think it has to do with the weight of the bike, as well as the efficiency of fully rigid. That bike also has a 36x 28 granny, so I'm pushing a MUCH harder gear than my mtn bike on climbs.

I think it maybe more a full rigid, skinny tire kind of a thing.

Cool vid..

The bike he's riding looks cool. I just wonder how durable it's going to be over the long term.
I looked it up. The Guide expert with rigid fork weighs in at 25.26 pounds.
That's only a few of pounds shy of my full squish and around the same weight as my 27.5 carbon hardtail.

I'd like to see a bike like this around 20 lbs. max.

so I didn't realize that you can't run an 11 speed road cassette on a mtb freehub so back to the drawing board. I'm hoping an XT cassette will work with my Ultegra shifters. 

Anyway, check this video out...the Santa Cruz Stigmata. The video looks miserably cold, but they are out riding so it can't be that miserable

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What size tire Jerry ?

I rode the thread trail last night w/ the new bike.  I think I need different handlebars if I'm going to ride a lot off road.  The bend in the drop on my current bars is a little shallow which puts a lot of pressure on my palm in a very small spot.  After 30 minutes buzzing the thread trail I wanted to chop my hands off.
Specialized sawtooth 700x42 is definitely not the preferred tire for rough and rooty single track...   

Those WTBs that ENOCH has on his bike look like the ticket.  How do they roll on pavement?  I presume you have many different wheelsets depending on the track.   

I just received my Hunt 650b wheels...  For the price they look like decent wheels... though not certain who makes the hubs.  I am initially going with the WTB Byway tires but based on my initial tire fitting they don't seem to have much more volume than the 700x42 that I am running now...  This may end up being a novelty setup that ends up going nowhere but if the wheels turn out to be decent I may pick up a 700c set.