First question, nuh, they just do some people who have come through Surf Lifesaving Club training also beat a paddling cadence with their feet, I reckon it upset balance and rocks the board more side to side an disrupts the glide but that's just me. Some of the competitive paddlers are superb athletes so it works for them>
Next knee paddling, depending on your ankles????? say what you can have your instep on the board with your feet flat or you can have just your toes on the board, I find the toes method gives me toe cramp and spastic toes which really hinder standing. Pop up is easier , but you need a bigger degree of flotation and stability more the flotation and finding the sweet paddle spot. You will get board bumps, surfers' knot or what ever they call them where you are , they are a blast from the past but can be problematical, and can develop sea ulcers if you break the skin, formal shoes also become difficult. They almost kept people for going to Vietnam. ( yeh board bumps)
BTw after my digression I have to tell you that knee paddling is faster in short burst as you have more muscle groups in the game. If you have Thoracic spine or lumbar spine issues proceed carefully.
