You don't have to be Einstein to realize that once you get that mind/muscle connection dialed in - you are going to be on the road to proper gains.
Far too many people go into the gym on a chest day, but as soon as they lie on a bench, they forget the fact that they are training chest!
Suddenly the only thing that matters is pushing a heavy weight from A to B... they have totally forgot that their single goal when walking through those doors was to trash the chest, and now they are just concentrating moving said weight with everything they've got, rather than focusing on maximum pec contraction, switching off the delts and the triceps, keeping the shoulders low, scapula retracted etc etc....
When clients first come to me, they are actually quite shocked at how little they can lift when I strip each exercise back to how it should be done to stimulate the target muscles... but this strength drop is only temporary, because in no time whatsoever, with the extra muscle stimulus that proper training creates, they now take giant strides forward - and more importantly, the bulk of the work is now being done by the target muscles...
And this applies to EVERY muscle group, not just chest!
That mind/muscle connection is key, and it isn't easy to get - you have to be aware of your body and have the skills to switch certain muscles off while focusing on the body part you are training.