Brembilla is Italian and the European 400m champion.  Hackett is Australian and the World record holder in the 1500m.  In the
sequences below, Brembilla is using a 6-beat kick, while Hackett is using a 4-beat.
 

1.
 

2.  This pose is standard in virtually all elite male swimmers - left leg downbeat while reaching with the left arm and turning to breath.  As
a recreational swimmer, I use this position as a kind of anchor, or base point.  This one is easy - some of the other positions are not.
 
 
 

3.  Right leg downbeat keeps the body driving through the water while breathing.  It's not real evident in this picture, more so in the next
one, but Brembilla swims with very deep left arm sweep and his elbow hardly bends at all.  Hackett uses a much more textbook high
elbow, bent arm catch on the left side.
 
 

4.  I believe this picture is crucial.  Both men delay the left downbeat that you see here until they get their right arms extended in front
of them and therefore into a streamline position to make the most use of this kick.  It may not appear that Hackett has kicked with his
left leg due to his body rotation, but in fact he has.  So both men are swimming "catch-up" stroke on this side, whipping their right
arm out in front very quickly to become streamlined and make use of the left downbeat as well as the final upsweep with their left
arms.  Brembilla's right leg is poised for its next kick while Hackett will now glide and omit two beats from his rhthym to conserve energy.
 
 

5.  First kick that Hackett skips.
 
 

6.  Second kick that Hackett skips - his left leg has drifted lower for balance, but he has not kicked with it here.  Brembilla obviously has.