Good question Hernan,
The short answer is "when steering is complete." But....
In reality, there are major 2 things that we see on both street and track that govern when you can finally crack open the throttle. But the book/video only covers one in depth, saving the other for detailing at a later time in a rider's skill level.
What is explicitly covered in TOTW2
Scenario 1: Getting your entry speed set just before or right at the turn in point. This allows a rider to begin their throttle roll as soon as the
steering inputs are complete.
Not explicitly covered in TOTW2 (brief mention)
Scenario 2: Trail Braking... If the rider has not arrived at the corner entry speed at the turn in point, the
steering input AND the remaining braking must be completed before a throttle roll can begin.
Many believe the perfect point where that safe entry speed and line come together is at the apex. So, many riders aspire to trail all the way to apex to set entry speed. The issue with that is, so many riders spend a lot of time at a very steep lean angle while on the brakes vs rolling the throttle. Pushing fronts and slipping rears and all…
Different riders have individual tolerances/abilities for braking, turning and visual skills. All of these skills take execution time, and it's those variances in individual times per rider that is the definition of "get back to the throttle as soon as possible". For some riders, it’s at the turn in point, for others… it’s the apex and for many more riders, it’s somewhere in the middle.
And...
It's fairly common for I/A group riders to trail brake deep in the corners on the track and especially while racing.
ASAP != Apex