If you don’t exhale on the most strenuous movement, your blood pressure pops up (your cardiovascular system is closed, and pressure in a spot compresses it) At best, you will get a headache. At worst, you will spring a leak. I do not buy the notion that you can not do the exercise without holding your breath. That is simply a result of conditioning, poor coaching and/or habit. You can and should break the habit. If you feel you must strain, get a mouth guard and clench against it.