When you begin the Long Stretch Series on the reformer, even when you're just doing Elephant, does your teacher remind you, "Thumbs with the fingers"?
Any time your hands are on the footbar, its for your own safety that you place the thumbs next to your fingers.
There are a few reasons for this. The main reason is, if your thumbs are on the opposite side of the bar, and your fingers are wrapped over, you could easily and accidently pick the bar up without meaning to. And you know what would happen if you lifted that bar up.... you would come crashing down. Flat on your face. Not a pretty visual.
Instead, you bring your thumbs forward, on top of the bar with your fingers. Your fingers are straight, not wrapped over the bar. You are just pressing down on the bar, not holding on for dear life. At Rhinebeck Pilates we say, "No 'death grip'".
Another reason for bringing the thumbs forward with the fingers is that its proper alignment for the wrists. If your thumb is on the wrong side of the bar, your wrist is torqued and could cause wrist pain.
Also, think of a plank position on the mat- your thumbs are pointed in the same direction as your fingers. Since the original pilates work was on the mat, and the equipment work came later, we base all movements on mat movements. Which means, thumbs with the fingers.
This goes for all the equipment during most exercises- the bars on the tower and when your hands on the seat or peddle of the chair.
There are a few exercises where do you take your thumbs away from your fingers, such as Rolling In and Out and when using the Push-Up bars.
For some exercises, it might seem scarey to bring your thumbs next to your fingers... but think about how much scarier it would be if you picked that bar up mid-exercise!
You can also check out my other post on this subject, "Hand Placement on the Tower Bars":