Maybe I have a wrong mental model of what's happening. In my undersytanding the yeast do their thing during primarythen when they have consumed the fermentables and/or reached their max ABV they either die or go dormant, and in eithe rcase settle to the bottom. So it seems to me that if all the yeast is at the bottom of the V, either dead or in 'suspended animation', then there will be little or no yeast in the transfer to bottle hence little secondary fermentation. But I am sure I have a over naive simplification.
@Victor Churchill, (at risk of others shooting me down) I think there's generally confusion between the naming convention for phases of yeast fermentation activity ('primary' & 'secondary' aka 'conditioning') and the naming convention used for different stages of the typical brew production process, (generally referring to stages when the brew is in 'primary FV', 'secondary FV', and in bottles).
I believe that primary fermentation more accurately refers to the early phases of yeast activity, which coincidentally happen in the primary FV. (During which, the yeast goes through multiple overlapping phases of consumption of available fermentables. Starting with the easiest to consume first).
Yeast activity will then drift into a secondary fermentation or ‘conditioning’ phase whereby it will finish off its consumption of earlier by-products - a 'mopping or cleaning up' phase. (Being the ‘secondary’ aka 'conditioning' phase of yeast activity, regardless of whichever vessel it’s contained within at that particular time).
Whereas a secondary fermentation brew 'process stage' occurs when new fermentables are added - usually when bottling - so that the yeast kicks off its first and most CO2 productive phase of consumption again, (but this time the CO2 forced into solution within the brew now contained within its final packaging, generating fizz).
A further confusion arises in that some brewers will transfer the brew from primary FV to a secondary FV, and before the yeast has completely finished all of its work. This is usually in order to remove a brew from sitting too long on old yeast trub, adding an additional optional brew 'process stage'. (In which case, albeit this will be in a 'secondary FV', it's not accurate to call it 'secondary fermentation', as such).
Most UK homebrewers don't rack brew to secondary FV, as it adds a degree of risk of infection and oxidation, with little of the benefit seen by commercial brewers. This is, I believe, to remove good beer from the risk of contamination from trub decomposition due to the immense pressure at the bottom of huge, tall commercial FVs, (a problem homebrewers don't usually have).
‘Conditioning’ occurs throughout the yeast fermentation activity beyond the initial phases, being that activity when the yeast is 'mopping up' or 'cleaning up' earlier by-products, and generally improving the brew. This can occur in primary FV, secondary FV (when used), but mostly occurs in the bottle, being amongst the latter phases of yeast fermentation activity.
Clear as mud (or maybe trub)?
Explained in greater detail by John Palmer in his excellent 'How to Brew' book and website, here...
Welcome to How to Brew - How to Brew/
Chapter 8 specifically on Fermentation, here...
Some Misconceptions - How to Brew