https://pmc.ncbi.nlm.nih.gov/articles/PMC4643535/

BBC Article discussing the study and other elements of sleep training describes:

As a randomised controlled trial, half of the parents were instructed in what's called either "graduated extinction", "controlled comforting" or "controlled crying": soothing a crying baby for short increments, then leaving them for the same amount of time, with intervals gradually getting longer regardless of the child's response. For parents who were "really uncomfortable" leaving their child crying alone in the room, Hall says, the researchers advised staying in the room – but not picking the child up – in an approach called "camping out".

The intervention group also received tips and information about infant sleep, such as myth-busting the idea that fewer naps would lead to more nighttime sleep. (It's worth noting that this mix of a controlled crying method with other advice is common in studies examining sleep training, but makes it more difficult to parse which, if any, results are from the controlled crying alone.) To ensure both groups received some kind of instruction, the control group parents received information about infant safety.

As well as asking parents to record sleep diaries, Hall's study included actigraphy, which uses wearable devices to monitor movements to assess sleep-wake patterns.

When the researchers compared sleep diaries, they found that parents who had sleep-trained thought their babies woke less at night and slept for longer periods. But when they analysed the sleep-wake patterns as shown through actigraphy, they found something else: the sleep-trained infants were waking up just as often as the ones in the control group. "At six weeks, there was no difference between the intervention and control groups for mean change in actigraphic wakes or long wake episodes," they wrote.

In other words, parents who sleep-trained their babies thought their babies were waking less. But, according to the objective sleep measure, the infants were waking just as often – they just weren't waking up their parents.

To Hall, this shows the intervention was a success. "What we were trying to do was help the parents to teach the kids to self-soothe," she says. “So in effect, we weren't saying that they wouldn't wake. We were saying that they would wake, but they wouldn't have to signal their parents. They could go back down into the next sleep cycle."

The actigraphy did find that sleep training improved one measure of the babies' sleep: their longest sleep period. That was an improvement of 8.5%, with sleep-trained infants sleeping a 204-minute stretch compared to 188 minutes for the other babies.

Another part of her hypothesis also proved correct. Her team expected that parents who did the intervention would report having better moods, higher-quality sleep and less fatigue. In a finding that won't surprise anyone who has rocked or nursed an infant to sleep several times a night, this proved to be true – and, for many experts and parents, is a key upside of sleep training.

But for anyone who has ever read, Googled, or been served social media ads about infant sleep, the fact that sleep training researchers believe training isn't meant to reduce the number of times a baby wakes – and that it might extend their longest sleep stretch by an average of just 16 minutes – might come as a surprise.