Being on the receiving end of a body triangle is easily near the top of the list for worst positions in which to find yourself. Off the top of my head, I can only think of a couple worse positions Anything requiring use of the term "teabag" probably ranks a little higher. But, the body triangle is bad; real bad. Why? Mainly because of the immense pain it causes.
If you've managed to avoid being put in a gnarly body triangle thus far in your BJJ career, that's good. Avoid them. If you must know what it feels like, then, rather than volunteering for one yourself, try putting one on the weakest, most inexperienced undersized and preferably sickly whitebelt you can find. Ask him (or her) how bad it felt afterward.
Alright, jokes aside - you need to learn defend, counter and/or escape from this position at all costs. Here are a few Catch Wrestling responses to the ol'body triangle brought to us by ISWA (i.e., International Submission Wrestling Assoc.) coach, Laren Umphelt. These approaches differ from the typical BJJ/submission grappling style. Good stuff.