The instructor didn't mention to have the person on their side to prevent the vagal response.
Sometimes it is impossible to lie the (conscious) person on their side so we do them sitting up at ~45 degrees, with the bed raised and trendelberged (so they don't slide off the end).
Lying on the left side is also supposed to keep the irrigation fluid from pushing the toxin into the duodedum, where it would be absorbed more rapidly.
I think most of the ones I have done have been done with the person semi sitting, unless they were completely out. If unconscious it is left lateral with the head down. We also use it for ETOH overdose, to ensure all the ETOH is out of the stomach. I have seen BAC go up from initial entry to ER to a few hours later, because they come in with a tummy full of beer, or whatever. Besides, I would rather pump it out than have them throw that foul smelling stuff up.
bob