There is nothing to explain here, but to make my point clear I'll give an example.

I've added the following lines to my xbindkeys configuration file:

#Increase Volume with Second Wheel Forward button
"/usr/bin/xte 'keydown XF86AudioRaiseVolume'"
   b:13

#Stop Increasing Volume with Second Wheel Forward button-Copy
"/usr/bin/xte 'key XF86AudioRaiseVolume'"
   Release + b:13

#Decrease Volume with Second Whell Backward button
"/usr/bin/xte 'keydown XF86AudioLowerVolume'"
   b:15

#Stop Decreasing Volume with Second Whell Backward button-Copy
"/usr/bin/xte 'key XF86AudioLowerVolume'"
   Release + b:15

#Mute Sound with Second Whell Down button
"/usr/bin/xte 'key XF86AudioMute'"
   b:16

The "Release" modifier works great when the selected language on my OS (Ubuntu) is US English, in fact it works with any English language. When I switch to Greek or Bulgarian (and probably to any other non-english language), it behaves like the "Release" modifier is not there.