My interpretation: you can prolong magic that has duration. If you whip a lightning bolt at someone it is a one shot effect (no duration) so it can't be prolonged. You have a lightning field block around you, then it has duration and can be prolonged. Notice that some damaging spells (like the suffocation blob spell) have duration, which could be prolonged. And blocks and manuevers can be used as means to cause damage, especially when used like grapples.