What is alternate arm and leg (bird dog)?

Alternate arm and leg (bird dog) exercise

1. Start on the floor, on your hands and knees.
2. Tighten your belly muscles by pulling your belly button in toward your spine. Be sure you continue to breathe normally and do not hold your breath.
3. Raise one leg off the floor, and hold it straight out behind you. Be careful not to let your hip drop down, because that will twist your trunk.
4. Hold for about 6 seconds, then lower your leg and switch to your other leg.
5. Repeat 8 to 12 times on each leg.
6. When you can do this exercise with ease and no pain, repeat steps 1 through 5 using your arms instead of your legs. Raise one arm off the floor, holding your arm straight out in front of you. Be careful not to let your shoulder drop down, because that will twist your trunk. Then switch to your other arm.
7. When holding your arm straight out becomes easier, try raising your opposite leg at the same time, and repeat steps 1 through 5.