2

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.

0

it means:

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.

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