With sendan, we mean that the saya has been wrapped with rattan string. Ribbed for your pleasure so to speak ;)
Just like a rayskin wrap around the saya, also rattan was a way to repair a split saya. There have been examples of leather, thread but but also metal bindings (semegane).
Since the semegane or the rayskin wrap were much more expensive, it's easy to figure out that the poor samurai used cheaper materials such as leather, thread or shown in the picture: rattan.
A great example you should definitely look into is Yagyu koshirae. One of the trademarks of this unique koshirae style is the sendan saya.
Anyway, in the picture I'm polishing a fully custom made sendan wrapped urushi saya. It's a real pain to polish in between the rattan channels and collection of threads (from coarse all the way up to fine) seem to do the trick just nicely :)
Sendan maki (???) can be translated as something like 'thousand channels wrap'. This describes a ribbed/beveled surfaces which you can find in many daily objects (No, not those kind of objects, you dirty monkey...? )
I'm actually talking about stuff like pens, chopsticks, pipes, porcelain and handles of tools such as saws.
The ribbed effect can be done by rattan, leather or simple thread. It can be defined as sendan maki, as long as there are channels created.
A big remark is that 'sendan' is not the same as 'inro'. I realize that both techniques have a raised/lowered surface effect but inro is a complete different story? I'll try to discuss that another time.
Anyway, to get back to the sendan:
In Japanese porcelain or lacquer-ware the effect is sometimes simulated by shaping the clay into a ribbed surface but often they would really wrap the wooden or clay object. Since one of the first layers of urushi lacquering is done with a urushi based clay (with tonoko pulver) any irregularities in the rattan wrap would be nicely covered so it keeps the channels but loses the sharp edges that can ruin the end result.
Sendanmaki is used as a decorative effect, a strengthening technique and as a way to improve grip on an object. For me it's beautiful to see that several techniques used in the craft of a Japanese sword were quite common in ancient Japan.
So theres a tiny bit more information about sendan. So now you know that it's been used on saya and several other items. Of course to again discuss swords: here's a picture of a sendanmaki tsuka?