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.)