It's not something I notice very often because I very rarely want that kind of acceleration. Others have noticed it though and some think it's a kick down switch, others think it might be a switch that overrides the speed limiter. I don't think it's a traditional kick down switch because the engine seems to kick-down without going that far. However I have noticed that if I push past the click then immediately release the pedal the car can buck around alarmingly so perhaps it's a 'super kick down'. Unfortunately it doesn't seem to be mentioned in the manual.
Assuming it is the same as the HR-V. I have just had one as a demo and can confirm it probably does both, overrides the speed limiter and possible kicks down the gearbox, but as andruec says, it kicks down anyway before the throttle reaches full travel. So not a conclusive test. A very quick dab, which is what I generally do, just overrides the speed limiter and doesn't give the gearbox time to react to a gear change.
The manual mentions "Kickdown" on page 395 but nothing about a full throttle stop.
Exceeding the Speed Limit Temporarily in covered on page 426:
The beeper sounds when the vehicle speed goes higher than the set speed limit with the acceleration pedal depressed completely.
The intelligent speed limiter resumes working once the vehicle speed goes lower than the set speed limit.