Using a secondary can be misleading when it comes to fermentation activity. In a secondary you can actually observe what is going on. In a primary such as a plastic bucket, you have no indication of fermentation progress beyond the airlock or overflow.
Using a primary (plastic bucket) and transfering after a week or two, I have noticed the airlock bubbling well, when no activity was seen prior to racking.
This included additional krausen production. In the process of racking, you do aerate the beer and thereby provide additional O2 for the yeast to continue their work consuming whatever is still available. This is of course if you are simply racking to a carboy which has not been purged with CO2 or you use a vacuum pump to keep O2 from being introduced.
Just my observations and my $0.02 worth. I am by no means an expert on the matter.
Salute!
