Lately, we have been focusing on ushiro ryote-tori or an attack from behind where the partner grabs both wrists with an overhand grip. Typically, this attack is first taught from static so that students can work on the basic movements and footwork. As the student becomes more proficient the attack becomes more dynamic and the students learn how to "draw" their partner in. In ushiro-waza training, spacing and timing are very crucial. Any problems with footwork or body positioning are magnified when the movement becomes dynamic. When the footwork isn't strong or the hand or body are out of position, the uke can collide with the student. This is typically seen when the uke steps on the nage's foot or when the uke's runs into the nage on the way behind.
On a basic level, in order to control the spacing and timing one needs to have mastered the footwork. The footwork enables the student to control the spacing. If one controls the spacing, it is easier to control the timing.