Can you hold a plank from every basic angle? Are you bored holding forever? What's the point if you can hold a plank for 2 minutes or more? Move on you've graduated to challenging yourself with more difficult variations.
First off, a plank is simply holding your body straight as a board. Think when you are at the top of a push-up movement and arms are straight. That's a plank. Most common mistake is letting the mid-section droop (which is a sign of weak core or abs, back and hips) or over compensating and rounding the back or piking the body up. Most common measurement of progress is how long you can hold the position.
Ok, so here's what's going on this video.
Shaun is holding a side plank on one elbow with one foot on a bosu ball, while holding his other leg up in the air to create more instability. With his other arm I have him doing a row with a light jumpstretch band.
Effect: With the progressive resistance from the band and the fast explosive pull, he's training to be powerfully fast as swimmers need. And being put into a position of instability, many various muscle groups are firing to maintain balance and control. Something to think about as when you are in the water there is nothing to brace you besides the water.
Remember this is only one aspect of a complete training program, don't just do these types of exercises. Be sure to incorporate strength, muscle stamina, cardio for heart and lungs, balance and coordination. Eat right and sleep well. Perform phenomenally!