Obliques (slant) muscles mainly come into play bending sideways from a standing position, twisting at the waist and supporting the trunk area of the mid body area. Side bends (DB or BB) affect the area strongly. Direct ab work (situps, crunches, leg raises, etc) to a lesser degree. Roman chair work can be excellent for the obliques and whole ab wall, when extra weight is applied. DB presses also hit the obliques, with any other over pressing press/jerking movement. Squats, DL's, ect, all get support from the obliques. Potential is of a very powerful muscle group, a very necessary unit that act as a stabilization factor with the abs and lower back.
SB1980 was correct that most BBer's will avoid working the obliques with any direct exercise. The idea is to keep the illusion of a narrower waist and a wider upper body. Building up the obliques will distract from that and give a more blocky appearance. Fat tends to settle there, and on the lower abs, for most men. Diet and some cardio come into play here.
Straight legged situps (a bad idea in the first place..try a bend in the knees, at least) and hyperextensions don't have a direct bearing on the obliques, but only in-direct action. The classic Greek and Roman male statues all highlighted the oblique, for what every reason. If you do want them developed, than try DB side bends. Do them holding a DB in one hand at a time, rather than going side to side with DB's in both hands (and always keep in line with the body, do not lean forward ,even a little bit).Just acts as a counter weight and lessens the work of the obliques. Also place the free hand behind the head when doing the exercise. 8-12 reps...2 sets, do not need endless reps or sets of this exercise. Good Luck.
