Video: Tomomi Nishikubo Versus Pure Trials Riders

 Once you get the pedal kick on flat ground figured out, the rest of it starts to make more sense. Not like "gee, I could do that now" but more of a "I see how what I know could translate into that with lots of practice and understand what they're doing as it relates to my knowledge."

 Pretty crazy how the sport has evolved, I started when we all rode modified MTB's that were 2 sizes too small.

What they call "pure" are really competition trials bikes designed for competitive events with the super long stem / rolled forward bars. It's to put a lot more weight over the front since competitive trials riding has evolved to riders hopping on the rear wheel a lot in events, the bar position puts them in a much more natural / neutral stance. You see the street trials guys using traditional bunnyhops to get up stuff...while the comp trials guys do a lot of "pedal ups", effectively using their pedal stroke to get the front up and landing on front to leverage the forward weight bias. The competitive trials bikes are really hard to traditional bunnyhop, super hard to manual and handle like garbage on two wheels...but once on that back wheel, they feel great.

On the opposite end of the spectrum, the "street" bikes would be worthless in a top level trials event where there are not run ups to obstacles and / or if you really need to get over that front end on an odd obstacle.

Rims brakes are generally a bit less prone to damage, have more lockup and are more direct feeling vs discs. You want that on/off feeling and hard lock-up, modulation is not a concern like street riders.

Knobby tires get great grip with the super soft knobs on a variety of terrain, concrete, metal, wood etc.....you'll also find a lot of the trials events are held offroad, with logs, rocks, streams, etc. thrown in. You could probably make the case for using a soft slick if you were just riding trials on concrete, but these guys do it all.

 you can probably tell from how awkward they look riding on two wheels (especially the mod bike, with the little 20" wheels) that they're designed to work best balancing on the back wheel. Most of the things you bring up are in service to that—the long stem, wider, grippier rear tire than front, etc.

I believe the main idea behind rim brakes is they can tuck in against the frame where they're much better protected than rotors. Of course you'll see street trials and 26" trials bikes with disc brakes too, so some of it may also just be tradition/if it ain't broke...

 @RadBartTaylor: beat me to it (and more thorough to boot!)

 I recall hearing once that a negative BB drop (i.e. BB shell higher than the axle plane) is also a trials-specific thing that makes those moves easier.

 @horkhork: Yea, it's call BB rise. Hydraulic rim brakes have been around for over 20 years on trials bikes now also. When set up right (good pads, right rim grind, good brake booster) they cannot be beaten for all out lock. 26" comp bikes pretty much only run rim brakes. Mods run a combination. I've recently acquired an inspired fourplay, man street trials bike are so hard to ride the same lines as I do on my Mod.

 @horkhork: Yup, puts the balance point more centered over rear wheel when hopping on back wheel....feels ridiculous when you are just riding around. Long and low front end helps too, get these bikes on the back wheel and you don't have to struggle to keep wheel up, they are very balanced....but they feel like horse&$*# otherwise though.

 Not an expert, but longer the bike the more you can shift your body weight, but comp bikes is like learning to ride a bike again as feels unrideable when you get on one

 If there’s a bike with no function, but built for fun only, it’s a trials bike, and that’s why I love them.