This is drill #8 in my swim book- Simple Swims: practices for balanced stable swimming.
This is an excellent drill to practice your balance -keeping your horizontal line, as well as stability-to stay tilted while bringing the arm forward, timing of the weight shifts-lead arm waits til elbows align. Maintain this before shifting your hips/weight.
Start with Front Float with a flutter kick then shift hips into Swordfish. Hold Swordfish while you bring your arm at the thigh forward allowing the Forearm to Drag. Continue a gentle Flutter Kick to help you stabilize as the arm comes forward. Hold your hips-not to let them "flatten" as you drag the forearm forward. Once the elbows have aligned, shift your weight from the left side of the body to the right side by simultaneously sending the right arm into the water, angling it in, down and forward. Use the weight of the body to transition from the left side to the right side and extend the arm forward.
Find yourself back in Swordfish again on the right side. Stand up, get a breath, then start again by going back into the Front Float into Swordfish now on the right side.
Use the drag of the arm during this drill to help both challenge and improve balance when dragging that arm forward through the water. Also, use this drag to get the feel of leading with the elbow during the recovery.