Sorry for being so late to this topic (I've just now read it), but it seems to me we are talking about the airspeed drop that occurs when you turn downwind from a sideslip.
GTE, I think the discussion that Shawne brought up may be valid for the Cessna spin that was the original subject of this thread.
Look at what airball posted regarding his incident:
We were circling while taking pictures and had done so many times previously in the same configuration (C150) often with less speed more bank and less stable winds. Suddenly with out stall warning buffet or any other symptom the a/c flicked over in to a full left-hand spin.
Then look at what djpil posted:
Only last week I was out in a C150 with a photographer circling at 1000 ft with various combinations of bank and sideslip (flaps up) thinking that an unwary pilot could get a shock doing things like that.
What both incidents have in common is using a C150 to take pictures. What djpil points out is that he had to use bank and sideslip while taking the pictures. One would assume the reason for this was driven by the need to place the aircraft on the proper ground track in order the get the desired photo.
When an aircraft is sitting at the end of the runway ready for takeoff with a 20kn crosswind, it is already in a 20 kn sideslip when it leaves the runway. When you land in a 20 kn crosswind, you are sidesliping 20 kn just to keep the aircraft on the centerline.
Rule of thumb (Shawne or GTE check me on this)
If you're sideslipping into the wind and then quickly turn downwind, expect a temporary loss of IAS roughly equal to your sideslip speed.
Remember, you are flying the aircraft with IAS speeds relative to a moving airmass. It's usually when you are referencing a fixed point on the ground (whether taking pictures, taking off, or landing) that you enter a sideslip relative to the moving airmass you are flying in, because you are concentrating on a fixed point on the ground.
The 2 Cessna C150 photo examples, the glider tug example, and the ultralight examples all seem to demonstrate this. Inertia comes into play when you consider how long it takes for the aircraft to recover the airspeed downwind that you gave up during the sideslip.