(CNN) — Erik Sweeney hurled his body toward the opposing team's quarterback, wrapped his arms around him and pulled him to the ground.
Seconds later, the back of his helmet socked the grass.
"It didn't hurt or anything," said Sweeney, a 13-year-old cornerback with the Wayland-Weston Thunder and Lightning in Massachusetts. "It just shocked like, it felt all fuzzy in the back of my head."
Despite that fuzzy feeling, Sweeney was raring to get back into the game. But a yellow light flickering at the base of his helmet caused a trainer on the sideline to stop him.
The light indicated that Sweeney might have suffered a moderate hit to the head. He was held out of the game, and within minutes, post-concussive symptoms began setting in.
"He sat out one play and then he began to get dizzy," said Sean Sweeney, Erik's dad. "Then he began to get the headache, and then it became evident that something was not right."
Sweeney was wearing one of a growing list of products marketed as "impact sensors" or "head impact indicators." They are devices -- small, relatively cheap, and often outfitted with a flashing light -- that are supposed to trigger when a blow to the head is too hard.
The product names evoke on-the-field action ("Shockbox," "Battle Sports" and "Check Light" are just a few), and they are marketed as an extra set of eyes on the playing field.
But what is that extra set of eyes really seeing?
Companies are quick to point out the devices do not see or diagnose concussions -- a common disclaimer in the burgeoning field of products to address head injuries.
Still, not many of these products have had peer-reviewed studies conducted on them to suggest whether they work.
"These technologies can be useful if used cautiously, as long as you don't overinterpret what they mean," said Dr. Jeffrey Kutcher, director of the Michigan NeuroSport Program. "It could be really dangerous to rely on this too much."
Experts who have tested commercially available impact sensors agree that more independent testing is needed before they are field-ready.
"One of the things we're interested in is when do they trigger and when do they not?" said Stefan Duma, professor and department head of the biomedical engineering department at Virginia Tech. "At what levels and in what directions?"
Duma and his colleagues are testing the impact sensors, and while their data are still preliminary, a recent drop test involving a few products -- conducted in front of CNN cameras -- suggested potential problems.
A crash test dummy head was outfitted with three sensors -- two with lights that are supposed to switch from green to either yellow or red after a big blow, and one that communicates back to the sideline when a hit is too hard -- and a high-quality helmet.
The helmet was dropped from various heights, approximating the force of blows that might be experienced on a playing field.
A drop from five feet, generating a force Duma described as similar to a jarring collision on a football field, did not trigger any of the three sensors.
"That's well into the range of concussion," said Duma. "You would definitely want an alert."
And at even higher reaches -- forces similar to running into a brick wall at full speed, or worse -- one system might activate, while the others did not.
The dubious test results, Duma said, underscore the need "to understand what (the products are) measuring, when they're triggering and when they don't, so that parents can have a better understanding of what information they're getting back."
Another concern raised by experts is the light. Could opposing players be motivated to "set off" or "light up" another player?
"I have a problem with anything that is going to, in real time, show a physical sign that someone's been hit," said Kutcher, chair of the American Academy of Neurology's Sports Neurology Section. "My belief is that's giving opponents a direct way to get someone out of the game."
One of Sweeney's coaches said that during his team's trial run with the sensors, he did not see opposing players motivated to set off lights.
What he has seen is the lights make the game safer, he says, because his players do not want to set them off.
"They played a game on Sunday, and there was a hard hit by one linebacker," said Mike Peterson, vice president of the Wayland-Weston Youth Football League. "He popped up and tapped the back of his head to get the other linebacker to check it. The other linebacker gave him a thumbs-up.
"That's the mentality we want the kids to have."
The issues raised by impact sensors are like an on-the-field version of the placebo effect.
Does having a light or some other indicator -- even if its efficacy is under question -- fundamentally change how children play the game? Does it begin conversations about head injury on the field that might not otherwise be had?
In Sweeney's case, the yellow flickering led to a later diagnosis of a mild concussion by his doctor.
"Without the device alerting somebody of this, he might have just gone back into the game," said Sean Sweeney. "I think the light is an awareness -- something to say, 'Hang on, pause, let's think about this,' versus sending him back into the game."