The toughest spot to brush in the entire mouth is the last tooth in all 4 corners.  A sure place I would always find plaque during cleanings was behind the molars, especially the upper molars. Because of the shape of our mouth and jaw, access to this area is limited.  The only way to reach behind your molars is to close down slightly and shift your lower jaw left or right, (depending on which side your brushing) until your toothbrush could comfortably reach.

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I thought if the toothbrush could bend slightly from side to side,  how much easier it would be to adapt around the curves and corners and ultimately remove plaque in the hardest to reach spots.  I was right!  That side to side bend was the trick! Easy access to the most difficult spots, behind the molars and that slight bend allows for adaptability around the curved canine areas, as well.