The anther arrangement in alfalfa has a fused group of anthers above the 
pistal with a single anther under the pistal that is fused to the anthers in 
the group.  This is spring loaded so that when a pollinator enters the 
flower, for the nectar, the bottom pistal springs free and hits the insect 
in the face.  This is what is referred to as tripping.  This coats the 
pollinator in pollen. As the insect crawls on in to the flower, pollen from 
a previous flower coats the pistal.  Honeybees do not like getting smacked 
and will learn to chew through the side of the flower to get the nectar 
without pollinating the flower.
Leaf cutter bees do not object to the smacking, and make better pollinators 
of alfala.
Dave

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