It is where you will do a course, usually not too long around 8/10 jumps and if you go clear you carry on straight into your jump-off (timed section)without stopping. If you get faults they will sound a bell after you jump the last fence and you must stop and not continue into the jump-off section. I find it quite hard for young horses or big courses as if you get a nice clear over the first jumps and go into the second phase, it can confuse them suddenly asking them to speed up and jump fences they have already jumped, it can also make it a very long hard course. At least when there is two or three rounds you can do your bit, come out have a rest prepare for the next bit and go in again. It makes the class a lot faster though as there is no waiting around and changing fences between jump-offs etc. It is usually done at BSJA shows in warm-up classes or at local level when they need to get through a lot of horses.
Our RC have a 2 phase next week and its a small sj course running straight into a small xc course, the last sj is out of the ring onto the xc course! or at least thats what they are calling it