Stocks were rallying Friday morning, then came reports of Ukrainian forces attacking an armed Russian convoy. The news breaks a string of positive developments on the geopolitical front -- including a temporary (at least) lull in tensions on the Russia-Ukraine border and Iraq Prime Minster Nouri al-Maliki stepping down -- that helped put the S&P 500 on track for its best week in four months and the Nasdaq's best in six months.

After trading as high as 1964, the S&P quickly fell to as low as 1946 before rebounding to near breakeven in recent trading. Regardless of the Friday morning squall, it has been a remarkable turnaround from the lows of last Friday morning, when S&P futures fell to as low as 1890, roughly 3% below current levels.

"Just when you think they're pulling the rug out, nothing seems to happen," says Howard Lindzon, chairman of StockTwits, who calls the "general rotation" from big caps to Chinese ADRs back to biotech a sign of a "healthy" market.

The U.S. market is buttressed by ample liquidity and low interest rates, "combined with the fact stocks aren't crazy expensive," Lindzon says. "Underneath it all - do you still want to put your money in Europe, or China or Russia?"

Indeed, U.S. stocks have become a "safe haven" asset of sorts in a troubled world, as I wrote here last week.

That said, this morning can be seen as a microcosm for the year to date: big swings in sentiment that have ultimately left major averages near unchanged. The bullish spin is that this is just market "digesting" the big gains of recent years. The bears, of course, say the bull market is running on fumes and heading for a ditch.

In the accompanying video, taped before reports of renewed fighting between Russian and Ukrainian forces, Lindzon takes a generally bullish stance. But he did express concern about the prevalence of ETFs, suggesting investors are being lulled into a false sense of security because of the diversity these investments purportedly provide.

"Have people taken the time to really understand what's inside their ETFs vs what's inside Apple?," he asks. "When the liquidity gets pulled as it always does, the hole is just as small as always when people push the 'sell' button. I foresee bigger problems because people don't understand what they own and may get surprised at the fills that come with their ETFs."

Aaron Task is Editor-in-Chief of Yahoo Finance. You can follow him on Twitter at @aarontask or email him at [email protected].

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