This all sounds rather cumbersome.
I find a doorkey poked through the tape about 1" from the edge allows you to rip the whole seam. You then need to put pressure on the lateral tapes, repeast the poke with the key and they also will tear.

Less cutting required, and you need never worry about losing or blunting your Stanley knife, or indeed a generic copy of it such as Homebase sell for about 2 pounds.
(username failed moderation, Tue 15 May 2012, 16:42, 3 replies)

Oh no, this special hook-thing was brilliant
The only problem was the blade in it was so sharp that eventually inquiring minds would start looking around to find other things that might fit in there, and sooner or later you've cut in half pretty much everything you own that's thinner than about 7mm
( emvee cruor deo cruoris, Tue 15 May 2012, 16:51, closed)

Plain Stanley rules for me.
You open it as your hand goes past the end of the tape strip holding the top flaps to the side, cutting it across it's width then you slip the fingers of your other hand under the flap at one side as your cutting hand loops back around and slips between the box flaps. You then run the blade the length of the gap until reaching the end whereupon you flick your hand back and find the blade in an ideal position to cut the remaining end of the tape holding the flaps to the side of the box.
The beauty of this technique is that it works for any thickness/strength of tape from cheap packing tape to Duck Tape without any change to the action and ensures that neither the blade nor anything else contact the contents of the box during opening.
(Me, I'm not... in any way intoxicated, Tue 15 May 2012, 17:26, closed)

You said box flaps
PFFFFT
( Snappyuk gets BIGGER with free sample., Tue 15 May 2012, 21:17, closed)

I read doorkey as donkey, and snorted into my drink
(jono_h, Tue 15 May 2012, 21:04, closed)