Working with the Cincinnati Marlins, they showed us a nice way to teach core stability during extension in breaststroke.
Why do it:
Creating a straight line during extension in breaststroke is more than just stretching your arms out front, you also have to have a solid body line.
How to do it:
1 - Start with 10 seconds of PLANK prior to diving in for the 1st 50.
2 - The 1st few 50s use a snorkel to keep the head down into position.
3 - Next, remove the snorkels and swim more 50s with planks focusing on extension and a stable body line.
How to do it really well (the fine points):
Keep the picture of the goal in mind and reach that position on each stroke. When performing the plank position, make sure you keep the hips in-line and the body firm.