The moment the blade enters the water at the front. It determines most of the drive in the stroke.
The catch is the key phase of the stroke: whatever you lose here, you won't recover during the rest of the pull. Done right it means entering far forward — torso hinging from the hips, lower arm long, and the blade fully submerged before you start pulling at all. That's exactly where the most common error sits: pulling too early while the blade is still half at the surface. It splashes, it's noisy and it produces almost no drive. The second error is stroking with the arm instead of trunk rotation. And exit the blade by the time it reaches your feet — beyond that the pull only brakes you and pushes the tail down.