Hi,
(I am just throwing this out there. I have not done extensive research or searched this topic thoroughly.)
I have a pet-theory hunch that the common story about how seamless shift gearboxes in f1 work is nonsense. The common claim that I have heard is that the gearbox engages two gears at once and then before it becomes a problem it disengages the previous gear. I think this is a half truth.
I think that there is a ratchet-like system where as soon as the driver actions the next gear, the next gear picks up some finger-like pawls. Just like if I was pushing a supermarket trolley, holding the handle on the LHS, and then someone comes up from behind moving faster than I am on the RHS and picks up the trolley and moves it forward faster than I was moving.
In this way the claim "both gears are engaged at once" is kind of true for a nanosecond but....is a misdirect. This also explains why drivers are told that changing down while accelerating is a no-no (I haven't seen this in years in any footage).
Again this is a not-thoroughly researched pet theory. I'd be interested to hear evidence as to what is really going on in a F1 seamless shift gearbox if anyone has alternate info.
p