What specific measures has Turkey taken to stem the flow of refugees to the EU?

Question

After the Cyprus gas spat this summer, in which the EU imposed some financial sanctions on Turkey, the latter has suspended the "readmission" protocol of Syrian refugees agreed with the EU in 2016. However, this measure in itself doesn't seem to have been that consequential to begin with (except perhaps as a deterrent):

After the signing of this agreement, the number of migrants arriving on the Greek islands decreased drastically: from 150,000 people per month in the summer of 2015, and 50,000 at the start of 2016, the number has now dropped to an average of 3,000 people per month.

However, the number of readmissions is low. Over the past three years, only 1,884 people have been returned to Turkey under the agreement, including 357 Syrians, according to data from Turkey’s interior ministry.

More recently, Turkey has requested EU support for resettling refugees back in northern Syria threatening that otherwise:

"We will be forced to open the gates."

So what (other) specific measures did Turkey implement that have stemmed the flow of refugees to Europe, measures that they could now undo, thus reopening the floodgates of Syrian refugees toward Europe?

(As a related note, there's currently a disagreement between the EU (Greece in particular) and Turkey whether the number of refugees coming over the Mediterranean has increased or not in the past few months. Also the Turkish effort to rid themselves of refugees recently appears at least in part driven by election losses.)

Answer 1

According to the Guardian:

A crackdown on migrants in Turkey, home to more than 3.6 million displaced Syrians, and renewed fighting in Syria appear to have spurred the jump. In August, Turkish authorities reported a sevenfold increase in those making the crossing into Greece. Nearly half of the 56,000 refugee and migrant arrivals in Europe this year have come through the Greek islands, according to the UN.

It seems Turkey previously allowed many migrants to stay. Based on the quote, it seems Turkey is making it harder for them to stay (going by the definition of crackdown). Unfortunately, the article doesn't mention what the crackdown entails.

So rather than old measures by Turkey now being lifted, it appears that previous tolerance has turned into a more active 'crackdown on migrants'. Based on Al Jazeera reporting, I think (Al Jazeera doesn't make the connection, this predates the reporting from Greece, but was reported after the spat in question) it may be related to arresting unregistered refugees and migrants causing others to move on to Greece:

A crackdown on unregistered refugees and migrants in Istanbul has seen more than 6,000 arrests, including that of Syrians, in the past two weeks, Turkey's Interior Minister Suleyman Soylu has said.

And a statement (also from Al Jazeera):

On July 18, the Interior Ministry's Directorate of Migration Management said in a statement that it was illegal for refugees with protection permits to stay in provinces other than the one in which they were registered.

Answer 2

Since the latest events have provided us with a "natural experiment"... what Turkey has been clearly doing now is turn a blind eye toward smugglers, which apparently dramatically lowered their operating costs, and so what they could charge their "customers":

"We had these migrants who arrived to cross Greece,” said a man who said his name was Semih. “It has become something allowed from our Turkish side. In the past we used to get them across for $200 or $300 per person and now we do it for $15."

He added: "Of course I see it as my duty. It's money that motivates me. I want them to go away from here [the migrants] so that Turkey's economy gets better. That's all that matters to me."

Another smuggler, who gave his name as Cevat, said: "Currently, migrants continue to flock here. Thousands of people, it's even hard to say exactly how many. Thousands of people pass through here from evening to morning.

"Our President Recep Tayyip Erdogan has given instructions on this subject, he has given his agreement for them [migrants] to leave. But on the Greek side, we are rather experiencing difficulties. Just a short while ago, right over there, Greek soldiers pointed their guns at us."