Product review (4 stars): Cheap Insurance; this type of product recommended by FD
This type of product was recommended to me by local fire department.  (They cannot endorse specific products).  Many fires occur around cooktops and dryers (lint buildup).  This product contains a fuse that's triggered by flame.  Looks like cannon fuse.  When triggered, it bursts open and disburses baking soda, and probably an anti-caking product to smother the fire.  It's held in place by a good magnet on the inside of your vent hood.  I cannot comment on performance so far, and hopefully won't need to.  It's going to ruin whatever you were cooking, but has a good chance to keep you from having to call the FD out.
You need one can for ever 2 burners.  This package contains 2 cans.  They mount 27"-34" above the cooktop.
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Update. Upped from 4 to 5 stars
First pair-had to assemble small parts on hanger.  A bit difficult.  Second set arrived assembled.
Still have not gone off.  FD says to call for any fire.