What do you do with that trail leg?
I think the trail leg is one of the most over looked and misunderstood movements you can do in bowling. If you watch a lot of bowlers you will see the trail leg (non sliding leg) doing all kinds of things. Bowling is just a series of very simple motions. But just like NASA bowlers need to consider the laws of physics when they bowl. You don't have know enough physics to put a man on the Moon like NASA but just enough to get by. One of the laws of physics that covers the motion of the trail leg and all the other motions we make as bowlers is this: Newton's 3rd law motion. "Every force or action has an equal and opposite reaction."
What that means to us as bowlers is this. When your ball swings from your shoulder it creates a lot of force that we can easily see. But there is also an equal and opposite force that we can't see unless we look for it. This force can been seen as torque and it is applied to your body. To best see a demo of this, sit down in a swivel type of chair or stool. Pick your feet up off the floor so the chair will swivel easily. Now, hold your ball straight down by your side. Then push your ball forward.
IF you are right handed your chair will start to turn to the right. You don't have to push very hard to see this. Now if you are say a 200 pound man and the gentile push you just gave the ball can rotate your whole body weight imagine how much force is applied to your body during a full swing of the ball. One of the most useful tools a bowler can use is the counterbalance. The better bowlers use it all the time. We put our non bowling arm out to help counter-balance our ball as we make our approach. If we have good timing the whole opposite side of our bodies works to counterbalance the ball's weight and motion. The trail leg can do a lot to counterbalance the motion of your ball as it comes through the bottom of your swing.
IF you use a little force and kick your trail leg back and to the left that is a very good counterbalance. Your leg is very heavy compared to most of the other parts of your body. That makes it a very good counterbalance. The heavy extended leg also raises your polar moment of inertia. This makes it harder for the torque of throwing the ball to upset the direction you should be facing. To demonstrate this go back to your swivel chair and hold your legs in tight to the chair and have a friend try to spin you around. Now stick your legs straight out and have him try and spin you again. He will notice that it is much harder to turn you now. I suggest when bowling that you try and keep your trail leg foot on the approach as you do this. Some people can do this better than others but it helps to have a deep knee bend. . If you try this you will see a change in your ball right away. You pick up ball speed and power with very little effort on your part. So how far to the left should your leg go?
The answer is as far as it can go and still keep your hips square to your target. You don't want to go so far that you open your hips and shoulder more than you should at the line.
If you would like to see some one the Pro tour use this just watch Parker Bohn. Bob Learn Jr (Mr. 300) literally stomps his trail leg into the approach. That gives him a ton of power.