Back Knee Bent In The Split Jerk?

Should the back knee be bent in the split jerk? Yes, but not necessarily very much.
We want the back knee at least slightly bent for 2 primary reasons:
First, a locked back leg pushes us forward rather than allowing us to absorb the weight straight down and therefore in balance.
Second, the straighter the back leg, the more tension it places on the hip flexors, which can mean the trunk is forced to lean too far forward, or the lower back is forced to hyperextend.
However, we don’t want it bent any more than necessary because that too can create problems, the most critical of which is that a significantly bent back knee means the back leg is unable to support much weight, so puts us in a weak position in which we’re prone to collapse or fall back away from the bar. It also makes it more likely we’ll be precariously up on the toes of the back foot rather than on more of the forefoot for better stability.
To keep it simple, you should be able to balance with your weight distributed evenly on the two legs, the back knee should always be behind the hip, the trunk should be leaned forward only enough to create the proper overhead position with the bar over the back of the neck, and the lower back should be in a neutral curve.
Use the push jerk behind the neck in split to feel the knees bending together to absorb weight straight down and maintain balance, then progress to split jerk behind the neck from split to increase the challenge and further strengthen the position.