So I've been doing hanging knee raises to target thee abdominal and while doing so I thought of moving the legs backwards in a back-kick kind of movement theorizing that since a forward movement targets the abs a backward movement should target the lower back. When trying this out I did feel my lower back being engaged and since this is a relatively simple movement I was thinking of incorporating this into my core workout. My question is this. Are there any dangers to this movement, eg an unnecessary strain on the spine, or unnatural movement of the hip or something? I haven't seen this movement in any guide for core exercises or even isolated lower back exercises so I was wondering whether there would be any possible injuries from this.
You may want to do back extensions instead. They're at least a more well known exercise that work the same muscles and are the lower back analogue of knee raises in a lot of ways.
First, please read Stuart McGill's work, and his ideas about core training.
Second, abs should be braced when you are doing forward flexion; and "glutes" must be contracted when you are doing extensions.
The move you are trying to do is done before, in a better way, and it is called "not back extension", but a "reverse hyper machine": https://bretcontreras.files.wordpress.com/2010/10/rev-hyper-ex.jpg