I believe you are referring to the "scorpion kick". This is a transverse plane exercise that focuses on rotational power through the hips for throwing, kicking, or swinging movements. It's great for multidirectional sport power development. The scorpion kick is an advanced exercise that requires stability in the prone plank position as well as a degree of coordination.
I would brake the exercise down into a few parts:
1. Start in prone plank position with one foot in the TRX (in single handle mode). note: TRX should be suspended 8-12" from ground, the further you pull the TRX away from it's anchor point the more resistance you will add to the exercise.
2. To ready the movement rotate the free leg under the suspended leg by twisting at the hip.
3. Initiate your rotation; swinging your free leg out from under the body and opening the hip to kick out the leg in the foot cradle.
4. Return to the loaded position with your free leg under the suspended leg.
I like to look to the direction that my hip is openning.
You will find the exercise in on Youtube with Pete Twist and Todd Durkin called TRX allstar trainer presentations. Or just follow the link (you will find the exercise at ~ 3:30): http://www.youtube.com/watch?v=T2SYOsCWG-I
Regards,