take a look at the iriminage approaches from Frank Doran sensei
http://www.youtube.com/watch?v=VnO70_KJki4
http://www.youtube.com/watch?v=i0Luo5a1Csw
don't cut down. legs muscle know how to deal with downward force (for example, gravity); don't know how to deal with upward force. float uke on his/her toes and shift his/her weight to his/her tail bone.