i do not mean actual sliding here friend, what i mean is when you hold the bar, picture in your mind your elbows and hands coming together at the same time, feels impossible which makes the chest activate to hold that, picture it all the way through the movement, as you press the weight press through your elbows, do a press the same way you would do a fly except forget what your forearms are doing, when you lift you dont focus on the weight you focus on the muscle, a curl isnt about where your hand moves its about which head of the bicep is contracting, see what i mean? every chest movement is effectively a shortened variation of a fly... in one direction or another, infact i would say the overall BEST builder for chest is manually assisted flys... basically what you do is you do the dumbell fly till the top motion then as you approach it your spotter puts pressure on your elbows to keep the dumbbells from touching, this means that tension is ALWAYS on your chest in every part of the movement as when the dumbells are directly above your chest there is NO tension on the chest at all and growth is all about time under tension
a good way to get this down would be to lay on the bench then do straight arm flys, nothing in your hand but just focus on doing them with your hands... put your hands together at the top and press them together then as you take your hands away from each other focus on keeping that feeling of contraction that you had at the top, then transfer that knowledge of how a contracted chest feels into your chest routine, god i wish i was better at explaining things :') i'll see if i can do a video or anything...
To add to what Jake is saying, combine his visualization of sliding the hands together with my concept of concentrating on bringing your upper arms together as the movement. This creates the proper mind-muscle connection. Too many people focus on pushing the weight up. As Jake and I are stating, if you focus on moving your upper arms and hands towards each other without moving your hands on the bar you will be activating your chest muscles. The weight to accomplish this is immaterial unless your goal is powerlifting. As you get stronger, increase the weight but maintain this focus.
John