Quick questions…When I am playing something on the ni no ito (hammer on/pull off), I get a lot of vibration on my ichi no ito. Is this common? My shamisen doesn’t have the adjustable screw ( name?) at the top/back of the Sao. I’m assuming this is why…???
Yup, the shamisen is supposed to resonate and that is why most of the songs don’t change key because you want the strings to resonate all the time.
Basically, if you play one pitch on a string the vibration will make other strings with the same pitch vibrate.
I’d call those the “sweet spots.” 
Time for a doughnut.
Ah you got some fat resonance going on you mean. If two people are of equal fatness and one of them starts wobbling his belly, the other one’s belly should start vibrating too. 
nice…like dueling sumos.
Wow Karl…I gotta hand it to you. Wobbling fatness as a metaphor to string vibration. That’s pretty stellar! (still working on SC, btw)
Neil, I’m surprised you are getting that much resonance if you don’t have Sawari on your neck. But yes as Karl says Shamisen is a resonating instrument and therefore it is truly suppose to vibrate on the “sweet spots” . It works well for music like hip hop or bluegrass or rock in which the key of the song very rarely changes. However we players tend to run into issues if someone wants us on a pop or jazz song with multiple key changes or transpositions etc.
Anyway I think you can look at our instrument as being similar to sitar or something. It’s a “one key at a time” instrument. The advantage of Shamisen is the fact that with only three strings and very large tuning pegs it is designed to switch keys in a few seconds.
If you ever find yourself playing in a band with tons of songs in different keys, be prepared to change tuning in between songs.
(you have a total of 5 to 10 seconds). It’s a bit nerve wracking at first but when you get use to it, it’s pretty darn cool!