Can anyone please explain if "doing" in "When doing something" is a base+ing verbal, or a present participle used as a verb in an elliptical sentence, or something else entirely. Here's an example of a sentence:
"When training, athletes build their endurance by practising their sports for long, exhausting hours."
And if it indeed is a part of an elliptical sentence, why is that so, and how is that different from this:
"Last weekend, I saw lots of ducks splashing around in the pond."
I understand that in this sentence splashing is a base+ing verbal, functioning as a modifier in a noun phrase.