Watching people ski over the weekend in the uncompressable sneet, an observer could see the fusion style of skiing people have in mind when they ask for combi skis.
The ski industry created the crippled mutant they call the combi ski, but teachers and coaches still envision skiers using it for one technique or the other, not both at once. Most models are skinny, built like either a classical or a skating race ski. Further down the line you might find progressively wider skis with softer and softer flexes intended to go further and further out of bounds.
The fusion, or sklassical ski is wider than a performance ski, and has a softer camber. A range from 48 to 52, maybe 55 millimeters at the tip is probably best, though you can assume the position and go through the motions on any ski.
The sklassical skier is one who has not developed strong technique in either skating or classical, but feels comfortable on skis as an eternal intermediate. The sklassical skater can manage V 1 and that's about all. In classical, the sklassicalist does not shift fully from one ski to the other. This is what keeps them from advancing in either technique. But the sklassical skier feels the drag of the kick zone when skating. This is what sends them to the shop in search of the mythical combi ski.
Sklassical skiers can be quite competent crossing terrain safely. Often they have good downhill skills, beyond the anxious fixed snow plow. Downhill skills are completely separate from propulsion skills. So, much as they might fudge it on the rolling ground and climbs, they can often come down with the best of them.
A sklassical skier could advance in skill by purchasing progressively more challenging skis. This is an expensive, long road, but if you just like to get out in the winter and learn things for yourself, it can be an enjoyable one. Lessons on technique-specific gear will advance your skills faster. But sometimes personal exploration yields deeper understanding of the principles. Or it can lead to well-solidified mistakes, repeated over and over. The danger is that the sklassical skier remains fixed in this mediocre wasteland, wanting things the equipment will only rarely provide, when aided by the most perfect conditions of weird snow. Sklassical works best when everything else works as haphazardly as sklassical.
Sklassical can be a good back-country technique. On one trip using waxable, double-cambered metal-edged skis, we skated on a Forest Service road because the wax wasn't gripping very well and the surface was firmly frozen. It wasn't much of a skate, since we had three-day loads in our packs, but we weren't going to move forward any other way. When we peeled off the wide road onto a narrow trail, we pulled back in to classical. It was more of an arduous stomp, since we were tromping through the mixed mess of snowpack and debris that followed in later January after the big 1998 ice storm.
On other occasions I have skated on wide, traditional-length exploring skis in upland beech forest and down along river floodplains. The snow was firm, taking an edge better than it held a track. In the best of conditions like this, one could take a real skating ski on some wild rides. I always worry about my poles in stuff like that, though, because real skate poles are way too long for comfortable fast descending through trees. I'd hate to bust my nice, light poles banzaiing like an idiot down some steep glade when I know it would be more fun with more turnworthy gear. Besides, I'm a wuss.
Real back-country skating is a complete topic all by itself. We may get some this year, with a good snowpack made dense by mild temperatures and plenty of moisture. Stay tuned.