Some users report strange hand flickering/twitching on medium graphics setting. This is caused by the combination of basic shaders enabled and atmospheric shaders disabled. It appears that on certain windlight settings, this combination will cause the flickering effect with the movement of the clouds.
Workaround: Enable atmospheric shaders or disable basic shaders. Both need to be either enabled or disabled in Preferences → Graphics → General.
If this doesn't help, disable Hardware Skinning as well.
The associated bug report is located at FIRE-14656.