Tani otoshi(谷落?) is one of the original 40 throws of Judo as developed by Kano Jigoro. It belongs to the fourth group of the traditional throwing list in the Gokyo no waza of the Kodokan Judo. It is also part of the current 67 Throws of Kodokan Judo. It is classified as a side sacrifice technique (yoko-sutemi).
with the tani-otoshi you place your leg behind both legs of your opponent while still holding him in you favorite komi kata and pulling the opponent backwards resulting in your opponent falling on his back and you falling sideways.
In the video, Gracie Jiu-Jitsu Basics Vol.1, Rorion Gracie and Royce Gracie demonstrate the clinch to foil a striker's game and take him to the ground. The animation below shows the use of a Yoko-sutemi technique for the takedown.