The catch position in the boat: vertical shin, long arm, closed hip angle, blade placed quickly and vertically. DRV model figures, mistakes and safety on the water.
The honest answer up front: the catch position is the foremost position in the rowing stroke: the shin stands vertical, the arms are long, the hip angle is closed, and the blade is placed quickly and vertically at the farthest point. That is how the technical model of the German Rowing Federation (DRV) describes it for sculling [1]. In plain words: you make yourself long, but you do not bend beyond the target.
Why is it so important? Because the whole drive starts from here. Reinhart Grahn of the DRV names as features of the forward lean: vertical shin, long arm, closed hip angle. He regards it as a mistake to row without any forward lean, because then no long stroke is possible, and equally as a mistake to row with "overreached lean": the shins should not be rolled more than a few degrees beyond vertical [2]. The model is a guide; depending on body size there may be deviations [2].
A distinction up front: this guide describes rowing in the boat on the water. On the ergometer there is no blade you have to place and no boat that can tip. Technique pictures from the ergometer are therefore not transferable 1:1.
How hard is it? You understand the position in one session. Until the blade enters the water quickly and cleanly in the boat, it takes many outings, in experience terms.
Roll forward until the shin stands roughly vertical [1]. You should not roll farther: shins not past vertical [2]. Milestone: your coach sees on video that the shin is vertical at the catch position.
The torso comes forward with a forward-lean angle, the arms in natural position, extended [1]. The hip angle is closed [2]. The shoulders are relaxed: reaching far out increases stroke length [1].
The wrists stay straight [2]. According to Grahn this is an objective feature that, for health reasons, speaks for straight wrists in the drive [2]. British technique descriptions also require that the lower back stays set and the body is "pre-stretched" [5].
Placing the blade happens directly from the rolling movement, quickly, vertically and at the farthest point of the catch position, exclusively from the arms [1]. The body position does not change at that moment [1]. The upper edge of the blade limits the depth [1]. Milestone: 10 strokes in a row with the blade in the water in one movement, without an air stroke.
Right after placing, your weight hangs on the handle, not in the shoulders [5]. That prepares the leg drive. Anchor: you hook in, you do not pull.
Film your stroke from the side in slow motion and pause the picture at blade placement. Is the shin vertical, is the torso unchanged, and is the blade in the water in one movement? If so, you have the three key points. Check the picture after every technique change, not only after weeks.
The foremost position in the rowing stroke, with a vertical shin, extended arms and the blade being placed [1]. It is also called the front turnaround.
Until the shin stands vertical. According to the DRV it should in no case be taken more than a few degrees beyond [2].
Quickly, vertically and at the farthest point of the catch position, from the arms, without the body moving [1].
The body position is similar, but the ergometer lacks the blade you have to place and the boat that can tip. Water rowing is therefore its own technique.
The Drive in Rowing: Legs First, Then Back, Then Arms