Dehydration headaches - you lose water, which is a primary component of plasma, the liquid portion of blood. This leads to less stroke volume, venous return, cardiac output (all basically leading to less blood pumped into systemic circulation by heart), which ultimately drops your blood pressure, and causes decreased systemic perfusion throughout your body, including your brain. There are mechanisms in your body (ie baroreceptors) which act to increase your blood pressure as well as redirect blood flow away from relatively non essential organs and send to your brain, but train long enough and you lose enough fluid that your biological responses will have a hard time keeping up. Also, I suppose with less blood going through the pulmonary circulation, you also may end up with either general hypotensive effects on respiratory exchange or even ventilation/perfusion mismatch - basically leads to improper oxygenation of the blood. You also have loss of electrolytes, which can lead to various adverse effects, since you need electrolytes for proper oxygen delivery as well as maintenance of the acid-base balance in your body.
If you hold your breath and tense or exert, the effects can be similar to what is known as a Valsalva manuever. That's basically what you do when you defecate - increased thoracic pressure against a closed glottis. This thoracic pressure causes a push in the pulmonary circulation, which leads to an early rise in blood pressure. However, like MC92MR2 stated, the blood gets backed up in the veins (decrease in venous return) because the returning systemic blood is unable to move against the high thoracic pressure (fluid mechanics - blood in vessels move with pressure gradients). Thus you get a big drop in blood pressure, which happens after about 5 seconds. So I don't know if you're actually holding your breath that long, but it's within realm of possibility that your blood pressure is being affected this way.
So yes, always hydrate yourself during practice (and hydrate yourself well before practice begins), replenish electrolytes if you can (teh gatorade), and remember to breathe.
I'm just a lowly MS1 (soon to be MS2 after path kicks my ass), and I don't think I did so well in physiology (the damn exam was so close to the April NAGA)...