Kite designer Ed Grauel in the early 1970's was not content with the Sled kite's performance even though he contributed greatly in making a much stable kite with the introduction of twin trapezoid vents.
The kite still had a tendency to wobble and yaw. He tried different types of vents with not much success. He then tried placing a canopy behind the main sail of the kite. 
 
This produces a drop in pressure which results in an increase in drive.
The canopy added to the Sled didn't improve the kite's performance. An idea came to him to sew the canopy down the centre from the top of the kite to bottom. This produced two air scoops instead of the single canopy.
This improved the kite's performance to the point that it eliminated the wobble and made the kite a stable flier.
The two scoops made the kite accelerate it's ascent after releasing to the wind. This rapid climb led Ed Grauel to name the kite a 'Bullet'.
The kite is covered by a patent but like Allison's original design Ed Grauel's creation sowed the seed for many variations.

This is similar to the slot effect of the jib on a sailing boat that concentrates the airflow over the leeward side of the mainsail and causes it to accelerate.
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