Since a lot of people are misreading the book, I'm going to have to spell this out explicitly.

For the forward half of a natural turn:

STEP 1 begins when the right foot passes the standing left foot, has it's middle (and perhaps the downbeat of BEAT 1) when the right foot is placed, and ends when the left foot passes the right foot on its way to step two. Commence to rise at the end of one means that the rise begins just as the feet are passing. Heel-toe footwork means that the weight has reach the ball of the standing foot before the feet pass, and the heel of the standing foot is just starting to leave the floor as the left foot passes it.

Step 2 begines when the left foot passes the standing right foot, has its midpoint approximaly when the left foot is placed (most likely the downbeat of BEAT 2). Step two ends when the right foot has closed halfway to the left foot. Continue to rise on two implies that rise is ongoing throughout the entirity of step two.

Step three begins with the right foot halfway closed, then sees the right foot finish closing and take weight. The end has not been formally defined, but would probably be when the right heel touches the floor. Continue to rise on three would imply that there must be rise during some part of step three - but nowhere is it specified how much. It could be only during the initial part, as the foot is closing. Or it could continue as the weight is switched from one foot to the other. Or it could continue even with the weight on the left foot - this has not been specified, so it is purely an artistic choice.

Also, keep in mind that the entire lowering process is really a prelude action to the next figure - the natural stopping point is on the rise, because once the body is falling it must move.