For advanced lifters, it's important to strengthen every part of your bench press. Board presses typically work great for the lockout, but doing board presses without boards can add a new twist to help kick-start your gains.
"Invisible board presses" are much more challenging since you can't rest on the board. You have to rely on your muscles to stabilize the weight. (Yes, this looks like a newbie cutting his range of motion short, but it's also a technique used by very advanced powerlifters.) This will build a tremendous amount of reversal strength since it takes a ton of effort to bring the weight to a dead stop and let it float in the air before you have to start it up again.
The Spoto Bench Press
Eric Spoto, one of the greatest benchers of all time with a 722 raw bench, coined these invisible board presses "Spoto" presses.
You can also pause at different heights depending on your sticking point. For instance, I like invisible two-board presses to strengthen the mid range to lockout portion of the lift. Remember, any time you do isometric work you're strengthening a few degrees above and below the point of the pause. This is why Spoto presses work the bottom end as well as mid range.
However, you should implement various heights in your training in order to gain strength in all areas. Again, right above the chest and about a two-board height are great places to start. You can utilize these with heavy weight for a few reps or lighter weight for a lot of reps.