The link to the Wiki Eductor explanation should have helped you...
The diameter of the section where your vac pickup has to be a little smaller that the rest of the piping, it makes the water flow faster through that section creating the suction. Also putting in a 'Y' instead of a 'T' angling it away from the incoming flow of water will help too.
I might have to do this to my front hose, just for water removal...I like it!
