Well, I will say I've heard people studying various arts who tell me their sensei says you can break/bend/damage blades quite easily with a jo. I've heard of different places where you're supposed to hit it to do this thing. I've also heard experienced swordsmen tell me their sensei says that hitting a sword on the (ji/mune/ha) (please fill in the blank) will be the safest place to deflect another sword or jo. I'm certainly not going to argue the point because it is much like being in a room with people of varied religions. Logically speaking someone in the room has to be wrong because they all seem to have different ideas as to the nature of reality. But logically they can't all be right. So I'll just say "My, look at the time", grab a drink at the bar and mosey my way out of the room...
I'll just say given my experience with stress testing swords myself I think they often, um, overstate the likelihood of success when it comes to doing damage with a jo.
Edge to side and the blade taking the impact to its side is going to have more of an issue. The cross section of a Japanese swords makes for the strongest direction being back to edge. The weapon wasn't made to "slap" people with the sides. But remember, when we're talking about hitting a sword with a jo we're not talking about a blade held in a vice. Nor are we even talking about someone holding a blade as hard as they can braced for a side impact.
I'm sure you could do some damage if one person held it out in front of them sides parallel to the ground. Then if they braced themselves well and held it as strongly as possible another strong shot to the side with a jo might cause a bend. Or even *possibly* a really strong snapping strike *juuuust* at the right spot.
And again there is the notion that all things are not equal. A thick blade without bo-hi and ample niku is going to be significantly harder to put a tweak into compared with a very thin modern "optimized for mat cutting" production blade with deep bo-hi. Those things will bloody near bend if you just hold them out sideways... Slapping a big nambokucho era robust daito with a jo will likely just dent up your jo...
Of course you can do some damage to a blade if the conditions are right. I'm not arguing that at all. But I think some might need a reality check when it comes to how easy it would be to do.
And the myth busters episode was using some of the cheapest production blades around. Fairly durable, but not example primo examples of the craft of the Japanese sword.
And fwiw we also tested out a Howard Clark L6 (very tough swords) against a 5160 (a very tough steel) production blade with a thick cross section. We used that blade because these overly thick 5160 blades are often described as being "super tough beaters". And besides, since they are relatively inexpensive it didn't put a dent in the wallet to get one just to tear up. Anyway, we literally bolted the 5160 blade to one of the supports of a large water tower. The first strike edge to mune put a gash in the mune. The second one ripped the bolts out of the wooden support (which shows how hard Tony was cutting) sending the blade flying (duck!). Once we got it bolted again Tony hit it a few more times on the mune. No bends, just some deep (a couple mm) gashes. We flipped it over and went edge to edge. The first one left a deep notch in the blade. The second strike cause the blade to snap but not where it was hit -- one of the mune cuts propogated and it snapped there further down the blade. We had two swordsmiths there watching (and an emergency room physician just in case). It turned out the 5160 blade was very poorly heat treated -- the grain was very large which meant it had been soaked at high temperatures for a long time. That makes swords brittle. But it took a *lot* of shots before the blade snapped.
I struck the Clark L6 bainite blade a couple times full force on the side with a oak jo while Tony was holding the sword. Even someone as strong as Tony couldn't keep the blade still while I did that. The L6 wasn't damaged, but the L6 blades are clearly an exception anyway. So it isn't a good test here since we knew full well it wouldn't bend (and so I didn't hold back at all). The more important factor was that Tony couldn't support it enough to keep it still with me slamming it with the jo. It is a very different scenario between holding a sword in hand and bolting it to a solid support.
I'm not saying you can't damage a blade with a jo. I'm sure a really talented fella could possible do some damage with a jo if the conditions were just right and the blade was thin or of low quality. But people need to realize that it is very difficult to damage steel. Especially a solid piece with a good cross section using just a piece of wood. Generally it is hard to damage something with a high tensile strength with something that is an order of magnitude softer and weaker. I'm just trying to point out that some of the jo vs. sword discussions where "breaking" a sword is mentioned is probably overstated. And most evidence I've ever seen for this has involved mogito being broken. Which is not exactly a good data sample given the nature of mogito...
A thin blade with bo-hi being struck by a jo? Yeah, sure, it is possible you'd bend it. A thicker blade? Increasingly unlikely. But breaking a blade? VERY unlikely. Of course these are just my experiences in actually striking blades against hard objects to the point of destruction. Maybe there are special spots you can hit them, or special ways of snapping at them with the jo. I'm not saying it ain't possible.
I'd love to see it done. Heck, I might even be convinced to donate a blade... Hmmm, maybe someday I'll set up a video camera and beat the hell out of an early Paul chen blade I've used for testing new polishing stones for years. It is a shadow of its former self but it might make a good test...