First of all, I'm talking about searing on a flat top. Which doesn't char. And it does keep in the moisture. Especially if you've got fat on the side of the meat. But the best reason to do this is not to hold in the water, but to get a consistently even temperature in the final steak.
It is not a fast process. The steak needs to be rested at room temperature for an hour. Then seared 2 minutes each side, maximum heat, seasoning before you turn it down. Taken off the flat top or cast iron pan, which you deglaze, and pour over the steak. Rested another 30 minutes. Return to a cast iron pan and put in the oven at 450F for 8 minutes for a 1" steak rare, plus 2-3 minutes to medium rare. (11 minutes for a 1" ribeye to MR).
What happens is that the outside is seasoned brown, well cooked and most of the juice stays in, but the interior fat melts down and you end up cutting through a piece of meat that is exactly the same color and temperature from the inside to the very edge of the outside.
If you do want to cook on a grill, obviously don't grease-stain it by flipping it a lot or trying to sear it there. Put your cast iron pan on the grill, sear it in that, and then finish it on the grates for flavor and char.
It is not a fast process. The steak needs to be rested at room temperature for an hour. Then seared 2 minutes each side, maximum heat, seasoning before you turn it down. Taken off the flat top or cast iron pan, which you deglaze, and pour over the steak. Rested another 30 minutes. Return to a cast iron pan and put in the oven at 450F for 8 minutes for a 1" steak rare, plus 2-3 minutes to medium rare. (11 minutes for a 1" ribeye to MR).
What happens is that the outside is seasoned brown, well cooked and most of the juice stays in, but the interior fat melts down and you end up cutting through a piece of meat that is exactly the same color and temperature from the inside to the very edge of the outside.
If you do want to cook on a grill, obviously don't grease-stain it by flipping it a lot or trying to sear it there. Put your cast iron pan on the grill, sear it in that, and then finish it on the grates for flavor and char.