As mentioned, quickshifters are designed such that you are to maintain consistent throttle opening when upshifting without using the clutch - I'm guessing you know this. As far as cutting power, I had that happen on my track bike with an aftermarket quickshifter and discovered that after upshifting I was not completely dropping my foot off of the bottom of the shifter. Thus, the minimal amount of pressure that I was still placing under the shifter was triggering the quickshifter's designed-in cut off. It was a random thing and took a while before I discovered the issue, as I had developed the habit with non-quickshifter bikes of keeping my foot rested under the shifter in preparation of the next shift. Had to change habit and force myself to drop the toe and keep it away from the shifter
