Winston's tracing of "2+2=5" in the dust on the table is fraught with great significance. He'd always insisted that, no matter how hard it tried, the Party would never penetrate the core of his inner being. And yet it has. Thanks to the brutal physical and psychological torture he endured at the Ministry of Love, he's internalized the Party edict that whatever it says is true, no matter how blatantly contradictory or absurd.
As Winston traces this impossible equation in the dust, he does so unthinkingly. And that was the whole point of the Party's breaking him, physically and mentally. The Party doesn't want Winston to think; it simply wants him to believe whatever it tells him to believe. And in order to achieve this outcome, it has had to plumb the very depths of Winston's soul, something he was always so certain could never happen.