"A transient hypertensive opening of the blood-brain barrier can lead to brain damage " http://www.springerlink.com/content/w5q7574041446w51/

Ron-
See if this is a new wrinkle for you. Temporary, acute rises in BP lasting only minutes or less can defeat the BBB and cause damage.

So what could produce this spike? More or less chronologically-
A constipated child straining at stool.
A boy in terror of Hitler or his drunken father.
Repressed emotion - especially anger or fear.
A constipated adult straining (hereinafter deemed "the Elvis effect").
A PWP whose autonomic controls can cause normal BP to rise rapidly when supine.
A PWP struggling to....(you name it)

What might prevent this? How about exercise? Or caloric restriction?

Nothing exists in isolation, so there is some interaction. The question is how much and when.