I agree with this, and I am thankful that you pointed it out. There are two (at least?) approaching collapses: dropping, and getting more than one object in the hand. The approaching collapse of dropping is present in many other situations, not just juggling. When I carry a bowl filled with water up to the rim across my living room I do not want to spill. I am avoiding to drop, but I am not juggling.
The second one, which is to avoid two objects in the same hand, is the one that is relevant. That is the approaching collapse that is part of the defining mechanism of juggling (specific activity). The way we solve it, to keep recycling throws in a pattern, is the relevant mechanism to determine if something is juggling (specific activity) or not. It is relevant because it is unique, the dropping one is not. The unique aspect of it is one object (or task) per hand (or control point) solved by recycling during the time of approach to a collapse. In order for juggling to be able to take place you need a situation where, when control is let go of, collapse does not happen instantly. It approaches it, and during that time, another task can be attended (or reset if you will). By doing this you can keep several tasks away from collapse, by interlacing the moments of control correctly, so that each time an object is approaching the collapse, it is “saved” by freeing up a hand (throwing what was occupying it) so that the approaching collapse is avoided. The object that was thrown away to free up the hand is now approaching a collapse, so the other hand is freed up to save that. The hands keep saving objects from collapse but each save triggers a new approaching collapse, cyclically. This is the mechanism that is unique to juggling (specific activity) and therefore is its defining property.
We are looking for the specific properties that are unique to juggling (specific activity), and can not be found elsewhere. It might seem arbitrary, but in reality it is like a game of comparison. Can property X be found in juggling and in something else? if yes, then it is not exclusive to juggling (specific activity), and therefore not a defining property. If property X is unique in juggling (specific activity), then we are probably on to something. In juggling (specific activity), the defining properties are a combination of:
- recycling of tasks (throwing and catching objects more in number than hands)
- avoiding an approaching collapse (never have more than one object in one hand)
There are other components present as well, such as avoiding to drop, but they also exist elsewhere, so we can not point at those as defining ones. If we do, then a bunch of non juggling (specific activity) activities will be encircled by our definition, such as carrying my cereal to the TV sofa.