NEW YORK (CNN/Money) - Stocks have been on an upswing since mid-March, but what makes technical analysts really happy these days is the fact that trading volume is gaining weight, especially on days the market rallies and, especially, among stocks that rise.

The Nasdaq's at its highest level since June of 2002 and the Dow is reaching for its January peak, but it's not fundamentals like the job market or corporate bottom lines that are really getting better.

Instead, traders point to technical indicators like expanding trading volume and something called the TRIN, which measures the relationship between the number of stocks that advance or decline in price and the trading volume that is associated with each category.

"I think we are in a recovery, but no one ever believes it ahead of time," said Phil Dow, director of equity research at RBC DainRauscher. "To me, it's pretty encouraging to see volume expand when the market is going up. And while the news is not improving, the market seems to be holding up."

The weeks before and during the war with Iraq yielded a sharp drop in stock market activity as traders sat on the sidelines, unsure of what would come next. But since the war was won, volume has been higher during rallies and lighter on days of selling -- a telltale sign the rally has legs, traders said.

"The volume expansion indicates that you might be seeing individuals, rather than just traders, getting involved," Dow said. "But, volumes in general are still pretty weak. Odds are that if this is the real thing, you'll see [daily] volumes of 1.6 billion and 1.7 billion -- and then it will be obvious to everyone we are in a bull market."

So far, the average daily volume on the New York Stock Exchange this year has been 1.42 billion shares.

In addition to the heavier volume on the days when the market is up, some experts also point to the TRIN -- an acronym for Trading Index -- which measures the volume relationship between advancing and declining stocks and could indicate overbought or oversold market conditions.

"There's not a lot of selling pressure in the market," said David Briggs, head of equity trading at Federated Investors. "Having sat here and watched and traded, you could tell something has changed. It's a bit of a reprieve. I think the market has to prove itself ... but this rally feels a little bit different. It feels a little bit better."

Briggs said the fact that the TRIN has spent a lot of time this year under 1, or breakeven, is a good sign. Because of how it is calculated, the TRIN's relationship to the market is inverse. A rising TRIN is usually bearish and a falling one is bullish.

"The more volume going into up issues, the lower the TRIN is going to be," said Jason Goepfert, founder and CEO of Sundial Capital Research. "There has been some buying pressure."

The TRIN 10-day moving average has been below 1 all but one of the trading days between April 23 and May 2, according Goepfert. Before this, the average hadn't dipped below 1 since Jan. 15.

But, while the indicator might have had a good run, Goepfert cautions that in the past three years, long-term low-level TRIN averages have been a good sell signal -- because of the bear market. Only a return to a bull market could keep the TRIN this slim for an extended time.