What you don't see

Here's the scenario, the offense catches the pull and immediately advances the disc up the break side.  The defense arrives.  The handlers execute a perfect dump-swing to the force side.  As the disc is swung, the receiver at the front of the stack takes a hard two steps forward toward the break side and then turns and cuts deep along the break side of the stack.  The force side handler catches the swing and makes a beautiful forehand huck to the endzone where the deep cutting receiver catches it for a score.  Everybody celebrates and talks about the great throw and the great cut.

Strangely, nobody celebrates the action that actually caused the score.  Not only is this action not celebrated, it isn't even noticed by most players in the game or on the sidelines.  Truthfully, the player who took the action may not even realize the opportunity he created.  If asked, he might respond that he was just making the cut that "felt right" at the time.

The key man involved in this scoring play and involved in most scoring plays is the man who vacates the position where the disc ends up being caught.  In the scenario above, the last man in the stack was the key to the score.  As the disc was being swung, the last man in the stack took 2 hard steps deep and then cut back in toward the force side handler.  The defender assigned to the last man in the stack never saw the actual deep cutter because he was too worried about his man catching the disc underneath.  By actively clearing the deep space, the last man in the stack created a scoring opportunity without touching the disc.

I'd like you to re-imagine the above scenario, but this time let's make the last man in the stack a more typically unaware receiver.  The offense catches the pull and immediately advances the disc up the break side.  The defense arrives.  The handlers execute a perfect dump-swing to the force side.  As the disc is swung, the receiver at the front of the stack takes a hard two steps forward toward the break side and then turns and cuts deep along the break side of the stack. The force side handler catches the swing and makes a beautiful forehand huck to the endzone where the disc is knocked away by the well positioned "last back" defender.

The "last back" defender will be congratulated by his teammates for superior vision and anticipation.  And, in a way, he deserves this praise.  He was aware enough to recognize an offensive opportunity and thwart it.  He was not, however, forced to make a choice.  The receiver he was marking (Mr. Typically Unaware, last in the stack) did nothing to keep his attention. Because his receiver was no threat, the "last back" defender was free to survey the field and anticipate the offense's movement at his leisure.

The effects of active clearing are not only felt on deep throws.  Any area of the field is susceptible to clogging due to improper clearing.  We have all been told that "you have cut away as hard as you cut in", but few of us have considered why.  A cursory look at any struggling offense will show you a group of receivers who do not create space for one another through active clearing.