The next round is on 1:35, but since it will take 12 x 100 before your interval brings you back to the "top," or :60, you have to endure 12 x 100 at 1:35.
The third round is at 1:30, so you only do two 100s and so on.
The beauty of this set is that anyone can do it, and modify it to their ability. If 100s at 1:40 are too fast to start out with, you may end up with a set looking like this:
12 x 100 @ 1:55
6 x 100 @ 1:50
4 x 100 @ 1:45
3 x 100 @ 1:40
12 x 100 @ 1:35
2 x 100 @ 1:30
And keep going until you "fail," or miss the interval.
Doing this set from time to time can also help you gauge your progress throughout the season. As you develop your endurance, your ability to carry the set through further than your previous attempt should increase, and you should get down to a lower interval and thus, more 100 repeats.
By the time you reach your fastest interval, you try and hang onto as many repeats as you can before "failure" to make the interval. The next time you do the set, try to beat your previous record by one more 100 repeat.
As a pace set, this is a great workout to familiarize yourself with your pacing capabilities. Starting out at an easy interval should be like beginning an endurance race: controlled and smooth with minimal exertion. As the set gets harder, you increase your speed and eventually plateau, repeating a certain time per 100.
This is your aerobic threshold, or "pace." Maintaining this speed for as long as you can will burn the optimum amount of calories while developing the endurance chops you will need to maintain that speed over longer periods of time.
This set also can be altered to incorporate different strokes, or even all four strokes if you want a well-rounded workout. Do each 100 as an Individual Medley: 25m butterfly, 25m backstroke, 25m breaststroke, and 25m freestyle, adjusting the intervals accordingly.
For athletes who want a longer set that's even more endurance-oriented, making repeats out of 200s instead of 100s would leave you with a set like this:
4 x 200 @ 2:45
3 x 200 @ 2:40
12 x 200 @ 2:35
2 x 200 @ 2:30
The versatility of this set has made it a favorite of coaches everywhere, and as painful as it can be for swimmers, they tend to respond to it for the mental concentration it requires.
Rather than turning their brains off for the duration of a mind-numbing endurance drill, athletes must follow the clock's revolutions as they work their way down the ladder, with the second hand serving as a direct indicator of their progress.
It's almost fun and definitely rewarding if you can figure out the pace clock and finish the set better than you did the previous time!