While by no means the most commonly practised “drop” technique, seoi otoshi has nonetheless been part of Aikido since its early days. A question often asked whenever seio otoshi is practised in the dojo is what is the difference between a seoi otoshi (shoulder drop) and a seoi nage (shoulder throw)?
Given the fact that both techniques involve the loading of the uke onto the back of the nagi (which is what “seoi”means), the confusion between the techniques is understandable. Its is also a technically trite (albeit correct) response to merely state that seoi nage is something more associated with Judo than Aikido.
From a technical perspective, the distinguishing feature of a seio otoshi is that the nage drops to one or both knees and stays there until the completion of the technique. By 
comparison, in seoi nage the nage completes the technique from a standing position. Further, a technique is still a seoi nage notwithstanding that the nage may have dropped to one or both knees and then returned to a standing position to complete it.
The below video includes demonstrations of both seio otoshi and seio nage by seniors at our dojo. The video is a compilation from various sources, including gradings and taninzugake (free form) practice.
Have a great week.
Aikido Warrior Dojo 