But for a move to be valid, the stock price action must be...

But for a move to be valid, the stock price action must be confirmed by volume. As technicians say, volume goes with the trend. Volume shows how much conviction investors have in a trend.

I've never really agreed with this theory although it is commonly cited. If a share price is moving up with volume, yes it means there is a significant group of investors out there who share conviction that the shares are cheap or good value. However it also shows that there are a group of investors who disagree, they are convinved that the stock no longer represents good value at the higher prices. There is no volume on a price rise without a counterparty who wants to sell.

Volume really shows how much disagreement there is between investors' convictions.
If the SP ultimately finishes up on high volume, then you can say those with bullish convictions outnumbered those with bearish ones. The good thing here is that due to the volume, its fairly safe to assume that those who disagreed have now all happily sold out.

But this isn't necessarily better than when, say following some good news, buyers and sellers both adjust their valuations upwards and the SP therefore moves up on relatively little volume. In this case the entire market agreed that the SP was cheap and adjusted their buy/sell prices accordingly.

Of course sometimes when the SP moves up on small volume it can just mean that there wasn't enough conviction on the buy side, so there wasn't enough meaningful buy side volume to entice the larger sellers to reveal their intentions. In this case they first wait to see if more volume appears on the buy side. These are the cases when a price rise on low volume can become transient.