Hi Phenix-Rider,
While I'm not going to say it wouldn't happen (I'd certainly look for it if in that situation), hamstringing might not have been as common as you may think. For one thing, you'd need to get around behind the opponent for that, unless using something like a specialist Yari (such as a Jumonji, or Chidori Jumonji Yari, or a form of Kama Yari, such as a Katakama Yari), or an even more specialised Kama. Part of this comes down to the cultural aspects surrounding combat, which dictates always moving forward, and thus not allowing that target, but other aspects that come into it are the higher-return targets that are available front-on.
One of the things to realise with Japanese armour is that it, in a number of ways, developed in stages. And one of the last stages to develop was the part known as Haidate (the thigh-plates, pretty literally "thigh shield"), which came about from about the 13th Century onwards. But, as a number of samurai found them cumbersome, heavy, and restricting, they were often not worn. In fact, a range of situations dictated that you would take them off (such as long marches, moving through water, being on horseback etc), which meant that the front of the thighs, as well as the inner thighs (with the femoral artery) was far more open based on the typical posture the warrior would be in (front facing).
Tell you what, though, here's an example of armoured swordsmanship, which includes some movement behind, and as such, some of the hamstring attacks you may expect, but also showing a number of other attacks possible against an armoured opponent:
And, just for fun (as this is a thread which has given some of the more unusual swordsmanship practices), here is another armoured section from the same Ryu (Yagyu Shingan Ryu), showing some of the defensive methods that could be used. This features their use of Jingasa, or "battle hat" (second half of the clip)....
One thing to realise, though, especially when it comes to battlefield (armoured) combative methods, is that despite it's reputation, a sword was far from a go-to weapon. It was a secondary, or tertiary weapon if your main one was lost or damaged. When it comes to the Shojitsu Kenri Kataichi Ryu there, one of their most prominent aspects is a movement refered to as To Dome (literally "sword stopping"), which is a finishing thrust down to the abdomen or groin in order to ensure that an enemy is killed. That, combined with the "searching" actions, and a few others, indicate to me that the dominant reason for this form of armoured Iai may have been for when going over a battlefield after the event.
When it comes to Shinmuso Hayashizaki Ryu, and the distance used there, yeah, it's an interesting aspect. You may note, though, that the long-swordsman is typically moving in to control (at least to begin with), rather than moving in to cut. That happens as a result of the short-swordsman's actions. The way I see these forms are that they are teaching both long and short sword methods at the same time - how to control with the long sword, how to move from that control with a short sword, and so on. The distance also lends itself to grappling Jujutsu responces, which are not available at a long distance. Tell you what, here's a few examples:
Oh, and just a small correction to an earlier post of mine regarding the Kage Ryu clip that no longer exists online. Although the practitioner who performed the Embu (demonstration) was a senior (experienced) member of the Ryu who had left to do his own thing, he was not a Shihan as I stated. I had forgotten that the Kage Ryu only have one Shihan, who is the person who heads the Ryu, and as such he was simply a member of the Ryu before splitting from them (Kaiin). Thanks to Mark (Langenschwert) for the correction and reminder!
