I asked him how he did it.
"Simple," he replied. "Instead of grabbing nage's arm on the way down [in preparation for a break fall], just extend both your arms in front of you, and follow them forward. Try it."
We switched, and instead of forward, I kept finding myself vertical, landing more quietly than my usual crash-and-slam, but somewhat awkwardly.
"I'm having trouble projecting out enough to go forward," I told him.
He made another suggestion, but by this time, the instructor noticed our conversation, and walked over to us.
"What's going on?"
I explained about the soft ukemi out of koshinage, and how I was having trouble mastering it.
"Soft ukemi?" the instructor chortled sarcastically, albeit with a hint of a smile, "Just hit the ground!"