Most effective, and can be used with the pants-ura-waza technique.
The simplest form of this technique is used on the modern adolescent who chooses to leave the upper edge of their lower undergarment exposed above the upper edge of the outer garment.
Blending is a quick two step followed by guiding the undergarment upwards with a quick jerk. This almost always results in a tap out, wide eyes, and the adolescent leaving the mat immediately.
The more advanced techniques occurs when your uke chooses the fashion attack know as a SAG. This is normally indicated by their pant like outer garment being worn with the top around the knees, these uke are easy to spot when running by the wide swinging motion of their legs instead of the normal up and down motion.
The technique is two fold: First two step and blend with the Uke this is a wider two step than normal to avoid the swinging legs with which the uke must move. Second as you come behind the Uke, give a quick downward jerk on the pants. If you are really good, you can choose to also apply the pant-ushiro-waza technique to the undergarments, or if you are just starting you may be able to get in a grundi-ushiro-waza, before the Uke self pins on the mat.
Note: monitor the color complexion of these Uke to rate the effectiveness of the technique, a red hue of the face most resembling a stop sign is an indication of uke's surrender, tapping rarely occurs.