Danaher says:

Strangles – Gi vs No gi: Strangles, and in particular, back strangles, are the single most devastating weapon in all of Jiu jitsu. Nothing else gives such certainty of success in application whilst maintaining your own safety from counters. Yet the two primary applications, gi and no gi, are very different from each other. In the case of no gi strangles from the back, you must use your head as a blocking wedge close to your opponents head. Your bodies will be aligned and the power of the strangle will come from the rotation of your elbow around his neck. The instrument of strangulation will be your wrist/forearm and bicep. This is rather thick and so often it can be problematic to penetrate under the chin and often requires trapping the opponents defensive arms prior to applying the strangle. In the case of collar strangles, things are very different. Now the instrument of the strangle is the opponents lapel/collar. It has a hard and very thin edge that slices under the jaw/chin like a knife and makes penetration to the strangle far more efficient. The collar/lapel is like a rope around the neck – an extraordinarily effective strangulation implement. In addition, unlike naked strangles, you want to form a perpendicular angle to your opponents shoulders and the power of the strangle comes from your bodyweight hanging off your opponent and your leverage leg pressing over his far shoulder. This puts all your bodyweight and leg strength into a strangle with a cutting rope around the neck – this creates a far greater degree of efficiency strangle than any naked strangle can ever hope to attain. Compare the two body positions in the photos and see the big differences in alignment, leg work and bodyweight application and understand that back strangles with a gi are a very different matter from strangles without one.