Here it goes:
When you clamp down to hold your breath and tighten your trunk muscles, you create a pressure gradient between the inside and the outside.
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When you try to hold your breath[...]
In other words, "when you attempt or make an effort to hold your breath[...]".
The Oxford Learners' Dictionary
to take strict action in order to prevent something, especially crime.
Based on this, I would assume that it roughly means the same thing.