volume goes up

I hope someone else can help. What I think he is trying to say is that ". . . a sizable amount of buying" causes volume to increase. Here, he is simply repeating himself.

I can't imagine what he is means when he says that ". . . volume goes up and stocks react strongly one way or another." Not a gramatical issue, but he is saying that an increase in the number of shares bought and sold causes share prices to increase or decrease. It may be a true statement or not, but saying share prices will go up or down is not the least bit helpful.

I can't feel that I've been the least bit helpful, but I do share your confusion.

He might be refering to throughput either at the stockexchange or his own brokerage company.
Obviously if there is a sizeable anount of buying there must be a sizable amount of selling. Stock (prices) react to this volume of buying and or selling.

Could be. I just don't feel that ". . . one way or the other" is especially useful information.

"Volume" in this sentence means the quantity of stocks traded. If you search for a stock quote, you will see "Volume" listed as the number of shares traded in a given day. "Thin volume" means a quite day with less than normal sales as opposed to "heavy volume" which means the opposite.

What's this "quite day"? Did you mean a quiet day?

Yes, quiet.

"Thin volume"? What a silly usage.

I don't disagree, but if you search for "thin volume" you'll get lots of hits on financial sites.

Yeah, in a big downturn, you can get badly bit by thinly traded stocks.