Your back can easily arch during any lower abdominal exercises that require you to lie on your back. Think about the hollow hold, where your arms are raised over your head while your legs are extended and raised at an angle to the floor. The closer your heels are to the floor, the more likely your lower back will want to pop away from the floor. This puts more pressure on your lower back while requiring less from your lower abs. Instead, keep your legs at an angle so your lower abs are engaged while pressing your lower back into the floor or mat (via Men’s Health).
You can also arch your back while doing planks. Even if your boot camp instructor tells you to drop your butt closer to the floor, be sure that your belly doesn’t sink so much that your lower back arches. This puts compression on your lumbar spine while your abs get a bit of a break. You’ll still be able to work your core and protect your lower back if you drop to your knees and tuck your tailbone (via Women’s Health). Think about the top of the pelvis moving back towards your spine (a slight posterior tilt).