WHILE / WHEN / SINCE -ING (Reduced clauses)

While / When / Since -ing 

No, no. Not sarcastic at all, but grievously out of date. Before ordering a taxi, and after ordering a taxi the noun-verb follows a preposition and is a gerund. Waiting for a taxi, patience is needed. Or Waiting for a taxi I dropped my wallet, are I think both participles,  

I think "when ordering" is equivalent to "if ordering" -- "when" indicates only that in this context, multiple concepts or facts coexist.

In contrast, "while ordering" implies that a second activity will take place concurrently, as in, "while ordering he could not help but stare at her nose". 

I believe Hugh is quite right to question the taxonomy here as gerunds. Rather, they should be treated as dangling participles (no subject for "ordering") or as a strained ellipsis ("When [one is] ordering a taxi, etc.").

Express time-related activities with reduced clauses

http://www.grammar-quizzes.com/8-12.html