I have been trying to grasp Wikipedia's convoluted explanation many times before but now I think that I finally undertood it. The downstep is simply a place where high pitch mora is followed by low pitch mora. If it is on the first mora then it is HLLL.. type, if on the second, then LHLL.., if on the third then LHHL.., etc. If there's no downstep then it is LHHH...
In other words, the intonations always starts low on the first mora (unless it has the downstep) and then rises until downstep is reached, from there it goes low again.
The interesting thing is that intonation continues on the particles that follow the word. If the word has downstep on the last mora, the particle will be low. If the word doesn't have downstep, the following particles will continue to be high. The link at the end of Wikipedia article contains a good example with "hashi ga".
