Quick review: In a jump combination, the landing edge of one jump is the takeoff edge of the next jump with no break in between. Since all jumps land on a back outside edge, under normal circumstances, the second or third jump in a combination is either a toeloop or a loop, since those are the only jumps that take off from a back outside edge.
Since the 2012-13 season, the ISU has allowed the half loop to be used in combinations. The half loop is a single-rotation, non-standard jump that takes off from a back outside edge and lands on the back inside edge of the opposite foot. It effectively allows the skater to “change feet” during the combination. If the skater rotates counterclockwise (as most do), this means that the half loop takes off from the back outside edge of their right foot and lands on the back inside edge of their left foot. This allows the skater to do a salchow (the most common choice) or a flip (rare) as the next jump in the combination, since those jumps take off from the back inside edge of the left foot.
On protocols, the half loop appears as “1Lo” and has the same base value as a normal single loop. It only counts for points if done in a combination. A half loop combo is considered a 3-jump combination; if a skater does a half loop combo in the free skate, they cannot do another 3-jump combination, since only one is allowed in the free program.
When badly done (sadly all too often), the half loop can easily look like a step-out or a mistake on a jump landing. The examples I’ve used here all have a triple as the last jump in the combination, but skaters can do doubles as well. The first jump in the combination can be any of the six standard jumps (toeloop, salchow, loop, flip, lutz, axel).