I'm a bit confused as to what he means by obiotoshi, and also the difference between the 2 variations of sumiotoshi. I've never heard of obiotoshi (presumably you grab their belt???), and I've seen so many different things referred to as sumiotoshi that it's hard to know what he means with the 2 variations.
Tori enters to the side of Uke-lower hand grabs hold of ukes belt, maintains tight control of the uke via belt, inserts knee behind uke and drops[otoshi ] uke backwards over his knee.Obi [belt ]otoshi [dropping motion ].This method ensures a good connection between Uke /Tori .It a strong waza. Tori can also take ukes neck and control the neck in conjunction with the belt hold-a variation-slightly severe method here. Cheers, Joe.