The loop is arguably the most recognizable and revered track element in Stunts. A tall roller coaster-like vertical loop, it is unguarded by walls and designed to be taken at speed. Rounding a loop is thrilling, and potentially quite risky if the track constraints make it difficult to keep the car straight. In competition racing, however, though, loops more often than not aren't crossed in the conventional way (Except when under OWOOT rules, which enforce it), but rather they span a wide category of shortcutting tricks.
In most cases, you lose time if you go through a loop the ordinary way, usually a couple of seconds. Therefore it is generally a good idea to go through the loop in a different, shorter and/or faster way that allows avoiding to drive through the whole surface. This is called "cutting" the loop, and has become a word of its own: loopcut/loopcutting. It can be done in several different ways, some of which will be described here:
The cleanest way of getting past a loop (from a racing line analysis perspective, that is!) exploits the ubiquitous collision bugs of Stunts, in a way similar to the slalom bug, but harder to perform. To do a bug loopcut, one has to shoot directly against the backside of the descending leg of the loop and hope the car will go through it. High speeds (above ~150mph) are required for pulling it off; however, the exact speed to make it work is completely dependant on how you are hitting the loop in a particular lap (often getting a few mph faster or slower will do the trick). The trick is also quite sensitive to car positioning - better results are usually obtained with the car pointing nearly straight and only slightly to the right of the midpoint of the loop. That explains the recommended technique for using the bug in NoRH - approaching the loop at speed with the car close to the track centreline and swerve gently, as little as necessary, to the right.
Another very common way of cutting a loop is to jump or drop from the right/inner edge of the loop after entering it. Inner loopcuts are often the best option when using the loop bug is not possible (unfavourable approach angle, or too low speed for instance). Not only that, they have advantages in relation to the loop bug, as the extra speed provided by the jump/drop can be very handy and offset the tenths lost in setting up the cut. The "jump/drop" duality is because the vertical angle at which the car leaves the loop can vary over a wide range with small changes on car positioning and steering input, thanks to the polygonal "bumps" on the loop surface. That makes it possible to leave the loop either pointing upwards (taking a high jump) or to force the car to drop downwards for minimal airtime. Of course, such sensitivity also means that when doing inner loopcuts at high speeds it becomes a difficult task to control the car trajectory, potentially demanding some RH effort.
Outer loopcuts are much like their inner counterparts, except done at the left/outer edge of the loop. The technical considerations are similar in principle, but if the track continues in a straight line landing back on the track can be a lot harder. Furthermore, drops are very difficult to induce, and jump heights tend to be higher - therefore, more time is lost on the cut. For these reasons, outer loopcuts are not as helpful as a way gain time by skipping a loop. On the other hand, they be used to do very high leftward-directed jumps without too much trouble (it is possible, but harder, to do similar things with inner cuts). Therefore, they can be very helpful if there is a reasonable way of landing back on without taking penalty time track after such a flight. Sections exploiting such loop jumps are a staple of track designing.
When not to loopcut
Loopcuts are commonplace in competition racing, and it is extremely rare that a track combo on its own makes the vanilla way of rounding a loop the best option. Some other factors should be weighed in, however. First of all, powergear is an important variable, as will be explained in the section ahead. The demands of NoRH racing also can force a driver to compromise and drive a loop in the conventional way, specially if the car speed is unsuitable for a bug cut, as inner and outer loopcuts often call for hard RH - in such situations, however, depending on how the track continues even plainly skipping the loop through grass should be investigated as an alternative before giving in.
Loops and powergear
Beyond the many sorts of cuts and jumps a loop makes possible, racers and track designers should consider powergear as a key factor in considering racing lines as well. Since the loop surface is nearly vertical at places, flexible powergear cars (Indy and Acura) can, once in their top gear, reach PG from almost any speed by driving around a loop without cutting it. For these cars, therefore, it can be very advantageous to drive a loop regularly for that purpose; and loops can modify dramatically the racing lines in an Indy/NSX race when placed early in the track.
Rigid powergear cars (Corvette and Ferrari) cannot reach PG with a loop in such an effortless manner, as they have to get well beyond their flat-track top speeds for PG to trigger. That would require driving around the loop several times continuously, increasing the speed a little further at every descending run until the 225mph limit is reached. While that may seem an unlikely way to save time, one of the most famous Rotoi tricks, performed on his latest ZakStunts race victory (Z47), is based on this general concept. A related example is provided by his disciple Ayrton on the Z92 winning lap.