poly which means many.
rhythm which means... ah come on people...
thus poly rhythm means... playing grooves ontop of grooves. it can be just within a drumset, like maybe bass snare. or within the whole band like maybe. lead guitar, rhythm guitar. its up to you.
like MESHUGGAH!!!
What i like to do is keep a 4/4 beat on snare. maybe on the 3rd count. and i do some funky 6/8 foot patterns.
I.e.
C|x-----------|----x-------|--------x---|------------|
H|----x---x---|x-------x---|x---x-------|x---x---x---|
S|-------x----|------------|x-----------|----x-------|
B|x-x-xx-----x|xxx-xx--x-xx|--x-xx-----x|xxx--x--x-xx|