Constraints are one of the most powerful tools for breaststroke...

Practice time is limited.
Coaching attention is limited.
Communication with each swimmer on every length is impossible.
Constraints help solve that problem.
Swimmers have clear goals and get clear feedback on every repetition.
They learn to create more propulsion with each pull and each kick.
They learn to adjust timing to hit different stroke count goals.
All of that can happen without constant instructions.
Effective constraints turn every length into a technical opportunity, even in a crowded group setting.
How?
Using a tempo trainer or similar device, give swimmers a slightly slower stroke rate than normal, and ask them to go as fast as they can.