Interesting discussion - I wasn't really trying to start anything; I just wanted to find out how common this procedure is and why I didn't know about it.
It is looking like this might be a "different in the US, different in the UK" type thing. I've searched the FAA FARs and AIM and they make no mention of switching to Standby. The
AIM is where I thought I'd find this sort of info, but it is pretty silent. The only relevant thing I could find after a quick search is:
From Chapter 4, Section 4-1-19 of the AIM
e. Code Changes
1. When making routine code changes, pilots should avoid inadvertent selection of Codes 7500, 7600 or 7700 thereby causing momentary false alarms at automated ground facilities. For example, when switching from Code 2700 to Code 7200, switch first to 2200 then to 7200, NOT to 7700 and then 7200. This procedure applies to nondiscrete Code 7500 and all discrete codes in the 7600 and 7700 series (i.e. 7600-7677, 7700-7777) which will trigger special indicators in automated facilities. Only nondiscrete Code 7500 will be decoded as the hijack code.
Searching the Bendix-King site yielded nothing of interest.
I believe (but stand to be corrected), that radar antennae sweep every 10 seconds, so presumably if one changed the transponder code within less than 10 seconds, the changing code would not show up on the radar screen. Would this not explain the delay that has been mentioned?