At the start of the first repetition, muscular contraction should be produced gradually, and should be slowly increased until the start of the movement is produced. Once movement at a slow speed has started, the level of effort should remain just high enough to continue slow movement. Do not increase the speed as movement continues.
When approaching the end of the possible range of movement, speed should be smoothly reduced to zero. Which does not mean you should relax in that position; but you should pause briefly in that position.
After a pause of one or two seconds in the fully-contracted position, movement should be started in the opposite direction; gradually and smoothly, and continued at a slow speed.
But you should not pause, nor relax, upon returning to the starting position; instead, a smooth change in direction should be produced without pause, and with no sudden or jerky movement. The exercise should be continued in that fashion until it becomes momentarily impossible to perform another full-range movement against the resistance.