Fixed rest starts when you touch
When a workout says 20 s rest, the clock starts when you finish the repeat. You then wait 20 seconds before leaving again. If your 50 takes 42 seconds, the full cycle lasts 62 seconds. If it takes 50 seconds, the cycle lasts 70. The recovery itself stays the same.
This is useful when the amount of recovery matters. It also works well across different speeds because every swimmer gets the same rest relative to their own finish.
A send-off includes both swimming and rest
On a 1:00 send-off, you leave at the start of every minute. Swim the 50 in 45 seconds and you get 15 seconds rest. Swim it in 52 and you get eight. Take longer than a minute and that send-off no longer works for the set.
The send-off therefore links pace and recovery. The faster you swim, the more rest remains, within reason. A send-off that is too tight can gradually turn a controlled set into a scramble just to make the next departure.
Information
Example: 8 × 50 m
At 45 seconds per 50, 20 s rest always gives you 20 seconds; leave every 1:00 gives you 15. At 50 seconds per 50, fixed rest is still 20 seconds while the 1:00 send-off leaves only 10. Same distance, same stroke, different load.
The right format depends on the purpose
More recovery can help preserve speed or demanding technique. Shorter rest can keep the work more continuous. A send-off can also teach pacing because going too hard early changes how much recovery you have before the next repeat.
Neither format is automatically better. They are different tools. The problem is changing one into the other without realizing that the set has changed too.
When the send-off does not fit your pace
If a prescribed send-off leaves almost no usable recovery, adjust it instead of destroying the set. Keep the logic: a regular departure that lets you execute the instruction. A 1:00 send-off might become 1:10 or 1:15.
Group practices often use different send-offs in different lanes for exactly this reason. The goal is not for everyone to leave on the same clock number. It is for everyone to train the same intention with an appropriate constraint.