Flash rips are "tricky and unpredictable" and can disappear almost as fast as they form.
Over the weekend more than 20 people at Maroubra Beach in Sydney's eastern suburbs were rescued after they were swept up in one by surprise.
Athletes competing in the Ironmen and Ironwomen series sprung into action, rushing out on surfboards to save the group that included tourists and children.
Coastal scientist Rob Brander said the volatile nature of flash rips can make them particularly dangerous, but pointed out one reassuring feature of these particular currents.
"They are over so quickly," he said.
What are flash rips?
Flash rips are a common occurrence in messy wave conditions, according to Professor Brander, who is often referred to as "Dr Rip".
Traditional rips sit in deep channels between shallow sand banks, appearing darker blue or green in colour, and can sit for days or weeks in the same place.
Flash rips develop suddenly, and only last for a few minutes.
"They are tricky and unpredictable ... and they pop up, up and down the beach," Professor Brander said.
"You might get a couple of big waves that break, the water level rises, that water pushes offshore and that's your flash rip.
"It might last a minute, and then it disappears, and all of a sudden you've got all these people in trouble.
"It's a really common mechanism for creating what we call these mass rescues, which is what occurred yesterday."
While people often blame sandbars giving way, Professor Brander said this was an "absolute myth".
"Sandbars don't collapse, it's rips," he said.
Can you avoid them?
"It's tricky", Professor Brander said, because "they are impossible to predict".
As they form so quickly they can appear like clouds of turbulent water and sand travelling offshore, and often start from the sandbar, where people hang for safety.
"You can't see it happening, and they can happen anywhere ... they can happen in the middle of the flags," he told ABC Radio Sydney.
On February 6, 1938, five swimmers died at Bondi Beach.
Several thunderous waves rolled into the beach in quick succession, knocking hundreds of swimmers off their feet.
The extra water quickly pulled back out to sea, carrying a crowd of struggling swimmers.
Professor Brander said only moments earlier those swimmers had been waist-deep in shallow water before they were bowled over by the freakish waves, in what became Australia's deadliest mass surf rescue, now referred to as Black Sunday.
What to do if you are swept up
No matter what type of rip someone is caught in, the universal advice is to stay afloat.
"Because if you're floating, you're conserving your energy, and that's giving you time to signal for help, which is the second most important thing you should do," Professor Brander said.
Swimming parallel to the beach to get out of a rip is sometimes advisable, but not always, given not all rips flow straight.
Staying relaxed, and not panicking, also helps avoid taking in additional water.
And because flash rips are over so quickly, they don't travel far.
"The rip is not going to pull you under, it's not taking you to New Zealand," he said.
"It's not taking you into shark-infested waters.
"And often rips circulate, and if you can ... float for a few minutes, you might actually find yourself back in shallow water on the sandbar."
If no lifeguards or bystanders are there to see the signal, surfers often will.
"Surfers do a lot of bystander rescues. I mean, we've done studies on surfers doing rescues, and they're doing a lot of rescues."
Or if you're really lucky, a few Ironwomen and Ironmen might be competing not too far away.