Question about House and Garden for Flowering

I haven't heard of nor seen any problems, but I'm a newbie grower...was wondering...I've been told by more advanced growers that they pull out all N by a certain time during flower. Seems like there's still a lot of N in H&Gs mix? Is it better to use less of the nute that has N in it? A friend said the drip clean helps with reducing the plants uptake of nitrogen???
Not having a problem currently...just kind of confused. Clarification appreciated, thanks.

Plants dont use as much N during flower but I would never cut N out completely! larger older fan leaves yellowing and dieing off later in flowering is normal as the plant starts taking N from the older leaves.