What is the secret to ensure hands are dropped into the right slot on the downswing?

This may come off as rude but…

There is no secret. Flashlight drills, mirror work, lasers, video camera try them all to establish feel vs real. Then keep on practicing and monitoring, short swings, slow swings, the lot.

Stop searching for secrets and you will have more chance of finding what you need.

The flashlight drill will at least make you understand the feel of coming down on plane, you’ll be amazed at what on plane feels like (very strange an illusion created by your own eyes) – most people’s light beam hit the wall one third of the way up – OTT.

Gotta,
Train your hands is the secret, alot of hard work is involved

What Toolish said. If there is a secret, it’s finding what it feels like “for you”, then work on re creating that feel as often as possible. For me, it’s 2 things;
1. A feeling of having my back to the ball at impact. That’s not what I do or look like, but it’s what I need to feel to avoid rotating my shoulders too early.
2. A feel of starting my downswing comparatively slowly. This also helps to prevent any tendency for me to start OTT, encourages a down & out path, and leaves me with power to accelerate through the ball.

But that’s me. For you it will probably feel entirely different. I found a camcorder very useful as I could video my swing, watch it back in slo-mo and see what needed work. As Bio and the other brains trust say, it’s an ongoing, never ending task and requires constant attention.

Hope you find ‘your’ secret.