Plank
Plank is the top of a push-up held still. In our Sun Salutations it lives for a single breath between standing and the floor — not as a long isometric strength hold, which is a different thing.
How to do it
Place the hands flat on the floor with wrists directly under the shoulders, fingers spread. Tuck the toes under and step the feet back so the body forms one straight line from heels to crown. Press the floor away through both hands. Engage the core, draw the front ribs in, and reach the heels back behind you. Head stays neutral, gaze just past the fingertips.
Common mistakes
Don't let the hips sag toward the floor or pike up toward the ceiling — both break the line and dump the work into the lower back or the shoulders. And don't bend the elbows; in this flow shape, the elbows stay straight. The lowering happens afterward, on the way to Cobra.