Suppose I've been practicing tennis for half an hour, and at this moment I am hitting the ball. Then there are two present events: my practice, P, and my hit, H. Of these events, H is a part of P. But H is distinct from P. After all, P started about half an hour before H. Hence H is a proper part of P. If the Axiom of Weak Supplementation is true, P has to have some proper part K that doesn't overlap H. But H contains all that is present in P. So, K is not present. Thus, there is a non-present event, and hence presentism is false. Assuming, of course, Weak Supplementation, which is the weak point of this argument.
I myself think that Weak Supplementation and Presentism are both false. The argument gets half-way: at least one of the two is false. (There is also the issue that the argument supposes a realism about events.)