Clutch shudder when setting off in 1st gear

[yaris2010RS]

something i noticed on my way home today tring to glide down a hill, possibally completly off topic but still has to do with clutch. i was driving down the road at crusing speed 100 KM/H and around 2,500RPM, so i depressed the clutch shifted to neutrel (no gear) and let go of clutch. i watched my RPMs drop to 1,000 then 500(my normal idle) then jump up to 1,500 and kept bouncing from 500 - 1,500. this only happens if i'm off the clutch and in no gear, if i'm holding clutch it wont bounce around. i took a video so if u dont exactly follow what i'm saying follow link

i'm sorry, its very bad quality and uber long, lol, i started messing around after a while, lol. and since my right hand (shifter hand) was holding phone made for a very bad experience. lol

http://www.youtube.com/watch?v=pEb1NY1xSXU