The theory behind this is not very complicated at first glance. A slow-fast pattern (for 2 jugglers) is actually a pattern in which each juggler has to juggle at a faster pace than the other. For example: if J1 makes 3 selfs between each pass (4-count), J2 would make only 2 selfs (3-count) or 1 (2-count).
The 2 jugglers then have to agree on a few necessary modifications. Since J2 is doing a 3-count, J1 will have to throw the passes to the left then to the right alternatively whatever their rhythm (ambidextrous or not). Thus, sometimes they will have to cross their passes (and sometimes not).
There is a slow side and a fast side to this pattern. Usually people say that
the slow side is the side where the juggling is slower.
For example: if J1 makes a 4-count and J2 a 2-count then J1 has the fast side and J2 the slow side.
If you would like to pursue this further, don't miss Johannes Waldmann article
in issue number 61 of Kaskade: Hobo Zwiefacher
You can also have a look at a few examples (which are not all that easy):
technofeeds, Alan's Anguish and the feed 3-count/ultimate.