I've noticed in myself, and am wondering what is going n, because when I push hard if I'm sitting on my chair - A thick Vein on my neck has a pretty big pulse, like it controls blood flow, and could be dangerous.
Pushing (when you go to the bathroom) can be associated with stimulating the Vagus nerve. This can lead to a rapid drop in blood pressure and heart rate and may result in fainting. If you loose consciousness from this, it is known as a vasovagal syncope episode. The act of bearing down in this way is known as the Valsalva maneuver (which does actually have a purpose in cardiology*). This information came from HealthGrades.com, similar information can be found on the Mayo Clinic's website (I didn't include it because it was a bit chopped-up).
From what I have read, bearing down isn't good for your head if you have a known brain aneurysm. Hence it is recommended that if you have a known aneurysm in your head, that you take laxatives (especially before surgery). If you would like more information on this, please visit the Stroke Association site.
*The Valsalva Maneuver is used as a treatment for a variety of conditions as well as a diagnostic tool for them (listed below). The maneuver's mechanism occurs when a person bears down and holds their breath. When this happens, pressure in the chest cavity increases and venous return decreases. This results in a very rapid decrease in heart rate and systolic blood pressure. It is because of this rapid decrease both in HR and BP that can cause dizziness or syncope not only when purposely preforming this maneuver but also when you are on the toilet.
The Valsalva Maneuver is used to treat SVT (SupraVentricular Tachycardia), traditional tachycardia (high heart rate), tachypnea (high respiratory rate), and A-Fib (Atrial Fibrillation) among others. It also has a wide range of diagnostic uses - not relegated to the cardiovascular or respiratory system - such as a pinched nerve, certain types of hernia, brain deformity etc.
Additional information on the Valsalva Maneuver can be found here and here. Please note that both of these links have a much better description of the exact mechanism of this maneuver than I could provide. Also, this YouTube link is very awesome! However, it was accidentally cut off at the end and there is no second part to it.
Hope this information is helpful to you, if you have additional questions regarding this, let me know and I will do my best to answer them.