20x100 purpose?

my coach has me do 100 repeats - but FAST.  It is to be able to hold a hard pace for longer.  If you just do long sets you don't get the speed work

I would ask the person who created the plan.

when I provide those to *my* athletes it depends on the pace but usually 20x100s are done @ threshold pace and the focus is to increase your speed at this intensity which in turn should make you faster at longer distances. The recovery set can have 2 fucntions, short rest sets to actively force your body to adapat at the precrived intensity, and long rest sets usually for workouts where the pace is rather taxing (i.e. VO2 max) and quality of each set is more important than quantity of reps.

Depending on the goals distance and fitness in general most of my athletes plans include 1 technique focus session, 1 threshold focus session (short sets @ T pace) and 1 endurance focus session (long sets @ steady to threshold pace). 

Building up short term speed helps build long term speed.  Goal for these should be to hit the same interval on every one.

I know for me, my benchmark IM set was 16 x 100 on X:XX interval (a tough one for me).  That was when I was the best IM swim shape because I could easily hit an IM pace of X:XX - 20 seconds and still finish fast (and my actual IM split was X:XX - 25 seconds)

Most has been covered but an added benefit will be to learn your pacing. You'll start crashing in pretty consistently at the same time which will help you get comfortable with your turnover and stroke rate.

But like most have said, speed & intensity.

You lost me on this one....

Would your tough benchmark set be, for example, 16x100 on 1:30 with maybe 5 seconds rest, or 16x100 on 1:50 with 20-25 seconds rest?  It sounds like the latter, though I would have guessed that the former would be a better predictor of a pace you'd be able to hold for an hour....

Brian 

You wanna swim fast? You gotta swim fast.

The shorted the distance, the harder you swim. 20 X 100 = very painful. Unless someone was standing over me - I would tap out at some point :-)

F'n A, Cotton.  F'n A.

I try to do at least one speed workout a week in the pool. My typical workout will be after warmup, 20 x 100 or 10 x 200 or ladders 300, 250, 200, etc. than repeat.

My focus on these workouts is to refine my stroke to find speed and smoothness. I go hard but if my form starts to fall apart I am either going too hard or am not concentrating enough. You do not get faster by thrashing madly. I will usually only focus on one or two aspects of my stroke at a time whether its keeping my head down or rotating properly or completing my pull, where my hand enters, how long I hold before pulling, etc.

The former.  The interval was actually 1:15.  It's not a predictor of how fast I would swim race day (i.e., wasn't a race simulation), but I knew if I could hold 16 x 100 at 1:15, I could easily hold 1:30s on race day (I was wrong, IM pace was 10 seconds slower at 1:25)