I know 4 different type of seamless shift gearbox. One what Honda use, one what Ducati use, one is efigear or pinion gear (maybe I should say 5
) which are used at bicyles and one is xtrac version of this system. There are videos on the internet. The last one looks not good to me. So I didn't interested with it much.
At Honda version, there is a rachet like system, there are struts sitting on the shaft. A ring keeps the strut in its place, and when ring give it a space it goes up by centrifigul force and locks gear to shaft. When this happened a small ball be pushed under the strut to keep it it's place to keep it locked with the gear when it gets reverse torque. Otherwise revers torque push strut back in it's place on the shaft and gear will be disangaged. In the same time active gear's strut's ball removed from under it. So this strut can be returned its place when it gets a revers torque. When you rev the engine higher and higher gear gets higher rev from lower gear activated shaft, it can automatically drive higher gear's strut it's place on the shaft, so it will be disangaged in the same way of upshifting. Even if this is not worked, they can push the strut back its place by using the ring I talked about. So this gearbox is able to shift both side with no problem.
Ducati made very clever engineering, It is something like a seamless converted dual clutch. I think Ducati version of this system can be used at road cars and motorbikes too. It is cheaper too compared to others. It is completely same with a ducal clutch gearbox. Just use roller bearing clucth instead of clutch. This roller bearing allow it to engage upper gear before disangeging lower so it works like seamless shift.
These systems don't need clutch for shifting but need rev matching. It is only power cut you got during acceleration if you have these gearboxes and it also can be eliminated at hybrid systems.