There are two variations of that movement. One is a kick and the other is a knee brace (tuck rt foot behind lt knee). In Bung Bo/Big Mantis it's done as a kick. I've seen Pong Lai's app for the move but I'll let them comment on it themselves. BTW, I have also seen the move done as just a cross step.
I've seen Little Mantis done as both a kick and a brace. A similar kick was also in another section of Little Mantis but was removed some time ago. Not sure why but it also appears in Say Lok. The difference in how it's executed makes it more viable than the kick in Bung Bo.
Now, the only usefulness I see for it as a kick is against a Chut Sing Ma where you shift your weight forward moving into the attack and stomping the knee of the straightened leg (ouch!).
Other than that I personally think it's weak and it turns your back too much to your opponent (unless it's a cover for a complete turn). Maybe someone else can jump in and convince me otherwise.
I think your use of kicking someone behind you is better than trying to kick someone along side of you. Actually, I just thought of a weird take-down scenerio for that move. It's a bit shakey though. Wow, now another one and this one makes more sense. Talk about hidden apps! Just need to try it on someone now. As you can probably tell I haven't researched Bung Bo or WL Big Mantis yet.
The knee brace can be a defend or an attack. We can discuss that too if you like.