Technically, there is always weight transfer in all groundstrokes, whether you've been moving forwards, backwards, whatever. That is because there's hip rotation, which helps to distribute your weight from one foot to the other. The only times where there's truly no weight transfer is if you lock your hips through the windup and forward swing, or if your trunk is so contorted that it cancels out the balance. Sometimes you see a person hitting a ball while still moving backwards. In that example, the weight is still obviously on their backfoot. However, as they initiate the forward swing, the weight transfer -- which is the delta, shifting, or change in distribution of weight -- is forward. Their linear momentum is going backwards, but their weight transfer is forward. (However the degree of weight transfer is still low since the front foot would be off the ground, thereby limiting the hip rotation.) Similar concept exists in football; when you see Favre or Brady throw off the back foot, their footwork enables their weight transfer to still go forward so that they can still throw the ball with authority. This is important because weight transfer places your center of gravity (i.e. forward weight transfer = center of gravity in front of you.) If your center of gravity is behind you, your balance will be off and it's very likely you'll hit the ball into the air. If your center of gravity is not really in front of you, then you'll have problems with making clean contact through the ball regardless of the extension in your swing. (BTW, a common problem with beginning 1H BHs. Even if you run toward the ball, if you block your hips by crossing over with the inside foot, you effectively "brake" your weight transfer and your center of gravity is no longer in front of you, even though your body is still moving forward.) All that said, none of the above actually addresses how weight transfer actually gives you power in the open stance FH, or how much weight transfer actually occurs in the open FH. But it's just to say that even in a true "sit and lift" FH (which itself also transcends stance), where there is much less weight transfer and the center of gravity primarily goes up during forward swing, there is still weight transfer.