One of the most common rookie mistakes when grilling or cooking steak is cutting into the meat to see if it has reached level of completion you want (IE: 'Med-Rare'). The problem with doing this is that it can ruin what would have been a great steak. It makes the meat tough and dry, especially when you still have to cook for awhile after making your exploratory cut. In the next paragraph I will explain in great and boring detail why this happens, if you don't care about the why just skip the next paragraph.
Warning: Pseudo Science Content (boring part)
This is why cutting into meat while cooking ruins the steak. With the outside of the steak effectively sealed by the searing process the moisture content in the steak exceeds the boiling point without turning to steam because the water can't escape readily, kind of like what happens in a pressure cooker. Both heat and pressure help to break down the collagen that holds the meat fibers together thereby tenderizing the meat further. If the pressure is suddenly released by cutting into the steak while still cooking in the pan two things happen.
1) Moisture will now exit the meat quickly as steam now that the pressure forcing it to remain liquid has been released, so in a matter of seconds a significant portion of the liquid is lost to the air drying out the meat substantially.
2) The sudden release of pressure and juice from the steak cause the fibers of the meat thicken and stiffen in seconds.
Viola, your steak just ruined your day.
Shhh! ... Don't tell anyone how this works and achieve grill "GodHood".
This technique is actually pretty easy once you know how. The secret to knowing when your steak is done is the firmness of the meat. A good reference for interpreting what each level of firmness translates to what stage of completion you have reached is found in the palm of your hand. Hold your hand in front of you, with palm up. If you follow with your eye from the flat part of the ball of the thumb down to the muscle pad at the base of your thumb which points towards the side of your hand. The firmness of this muscle pad felt when you gently poke this spot with the finger of your other hand, will pretty closely approximate the stage of completion known as 'Med-Rare'. The muscle pad next to the last one on the side toward the index finger represents 'Rare' to 'Blue-Rare'. The muscle pad on the other side of 'Med-Rare' will match 'Well-Done'. With a little practice you will soon be a 'Steak Master'. Keep in mind that different cuts of meat will react differently to the cooking process and you will have to adjust this technique appropriately. Don't be afraid to practice and experiment. Don't forget to let your steak rest on a plate for a few minutes, about 5 minutes per inch of thickness. This will allow the meat fibers to relax and the juices to stabilize and rehydrate entire steak so that it will be fork tender, juicy and full of flavor.
Where to go from here.
Now for some of you out there who feel somewhat overwhelmed at the idea of the entire process of choosing and cooking a great steak, take heart. I will be posting an instructible soon covering everything, from the trip to the butcher shop or meat counter at the grocery store, to knowing what cuts are the best for a given circumstance and every step along the way to becoming the hero of your block, the 'go to' gourmet when it comes to cooking the best steaks you've ever had.
Another Delectable Instructible from Fuzzee Dee