As we discussed in the Advanced Exercise, it's absolutely critical that you have strength and stability in the lower half of the body for the one plane swing.
You're creating most of the club head speed with the rotational force of your upper body, so the lower half has to provide a lot of stability throughout the shot.
If you're generating a lot of force with the upper body and the left leg gives out and you start to slide into it, you're going to start missing shots. You won't be able to swing with the authority that you could if you had a good stable base.
We have two great intermediate exercises for this, and you can do them anywhere. You don't have to go to a gym. You can do this out on your lawn, in your house, anywhere you want. It's excellent for strengthening the lower half of the body.
Split Legged Jump
Switch legs in the air to strengthen each leg individually
The first exercise is what we call a Split Legged Jump. You're going to put your hands over your head and scissor your legs about three feet apart, with one in front and the other behind you.
Now leap into the air using just your lower body to generate the upward motion. While you're in the air, switch legs and land with your feet in the opposite positions. Do three sets of 10 reps. This will strengthen each leg individually.
Again, get your hands over your head, get into a good squat position with your hips underneath you, and just explode into the air as hard as you can. Do three sets of 10, with about a minute of rest between each set.
Now Both Together
Once you feel comfortable with the Split Legged Jump and you've worked each leg individually, you can progress to a second exercise that takes the same idea of using your lower body to generate force and explode up into the air, this time working both legs together.
Start with your normal shoulder-width stance, then squat down and jump into the air as high as you can, pulling your knees to your chest. Do three sets of 10.
Leap with both legs together, pulling them to your chest
Again, throw your arms up into the air and leap as high as you can, bringing your knees to your chest with each jump. You can take a little hop between jumps to help develop a rhythm, but don't stop. Keep going constantly for each set of 10.
This is a plyometric exercise that uses a lot of explosive power, so it's not something you should do every single day, especially if you're grouping it with other work. Three times a week is plenty.
Do it in the morning to wake up these muscles after you warm up, and it will really help build strength and give you stability and confidence in your lower body. You'll know you can swing as hard as you want with the upper body, and you'll have all the support you need.