The phrase Entzug ist Ereignis means the withdrawal is enowning. This passage from What Is Called Thinking brings together the withdrawal, das Sichentziehende (the “self-withdrawing”) and man's role in it.

We said: man still does not think, and this because what must be thought about turns away from him; by no means only because man does not sufficiently reach out and turn to what is to be thought.

What must be thought about, turns away from man. It withdraws from him. But how can we have the least knowledge of something that withdraws from the beginning, how can we even give it a name? Whatever withdraws, refuses arrival. But--withdrawing is not nothing. Withdrawal is an event.

That "event" is Ereignis in the original. Whatever withdraws, conceals itself--it is the lethe, the hidden.

In fact, what withdraws may even concern and claim man more essentially than anything present that strikes and touches him. Being stuck by actuality is what we like to regard as constituitive of the actuality of the actual. However, in being struck by what is actual, man may be debarred precisely from what concerns and touches him--touches him in the surely mysterious way of escaping him by its withdrawal. The event of withdrawal could be what is most present in all our present, and so infintely exceed the actuality of everything actual.

Now the self-withdrawal.

What withdraws from us, draws us along by its very withdrawal, whether or not we become aware of it immediately, or at all. Once we are drawn into the withdrawal, we are drawing towards what draws us, our essential nature already bears the stamp of "drawing toward." As we are drawing toward what withdraws, we ourselves are pointers pointing toward it. We are who we are by pointing in that direction--not like an incidental adjunct but as follows: this "drawing toward" is in itself an essential and therefore constant pointing toward what withdraws. To say "drawing toward" is to say "pointing toward what withdraws."

This final bit explains why Der Entzug ist des Daseins.

To the extent that man is drawing that way, he points toward what withdraws. As he is pointing that way, man is the pointer. Man here is not first of all man, and then also occasioanlly someone who points. No: drawn into what withdraws, drawing toward it and thus pointing into the withdrawal, man first is man. His essential nature lies in being such a pointer.
P. 9

Another explanation of the Entzug ist Ereignis might be found here:

...to giving as sending there belongs keeping back--such that the denial of the present and the withholding of the present, play within the giving of what has been and what will be. What we have mentioned just now--keeping back, denial, withholding--shows something like a self-withdrawing, something we might call for short: withdrawal. But inasmuch as the modes of giving that are determined by withdrawal--sending and extending--lie in Appropriation, withdrawal must belong to what is peculiar to the Appropriation.
...
Insofar as the destiny of being lies in the extending of time, and time, together with being, lies in Appropriation, Appropriation makes manifest its peculiar property, that Appropriation withdraws what is most fully its own from boundless unconcealment. Thought in terms of Appropriating, this means: in that sense it expropriates itself of itself. Expropriation belongs to Appropriation as such. By this expropriation, Appropriation does not abandon itself, rather, it preserves what is its own.
P. 22

Ereignis is translated as Appropriation in this text.  ¶ 10:00 PM