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Slide tackles should be a relatively rare element of your game. A defender should only go to the ground if there is no possibility of defending the play while standing up. If the offensive player is going around you and you have no support, it might be appropriate to go to the ground to stop an offensive charge and give your support a chance to catch up. A good slide tackle usually takes the ball out of bounds, changes possession, or otherwise clears it. Making an unsafe slide tackle warrants an automatic red card and ejection from the came, with the possibility of further punishments from the league or your coaches. Only use the maneuver if absolutely necessary. The best time for a slide tackle is when you and your opponent are running parallel, toward the ball, and that you're going to lose possession very quickly if you don't make a move now. If you can't get a foot on the ball while you're in a standing position, and you're about to lose ground, it's the perfect time for a tackle. Never make a slide tackle from behind. Tackles from behind or from head-on are very dangerous and the possibility of a penalty is high. It's possible to do it correctly in some situations, like if you're both going for the ball but neither is in possession, but not recommended unless you're sure you'll be able to get to the ball first. The ideal moment for a slide tackle is just after the player in possession has kicked the ball, creating separation between that player's body and the ball itself. That will give you the most room to maneuver and get your foot in the way. Keep your eye on the ball and watch absolutely nothing but the ball while you slide through to tackle. A good slide tackle should be focused on nothing but getting the ball out of the way. Before you decide to take it to the ground, do a quick double check of your emotions. You don't slide tackle a player, you slide tackle the ball to get your team control. Targeting a player will get you ejected and possibly kicked off the team. Lead with your dominant foot–the foot you use to kick with–and hook your leg into the ball, making contact with ball first and foremost, and sliding as softly as possible onto the ground, through the ball, and across your opponent's path.  A proper slide tackle is just like a baseball slide. You shouldn't leap up into the air and come crashing down on your hip, or you risk hurting yourself. Drop the shoulder closest to the ball and slide with your leg extended. Never slow down before taking it to the ground. You should accelerate into a slide to make sure that your leg follows through with the motion, clearing the ball, not just falling flat. The legality of the slide tackle depends totally on whether or not you contact the ball first. If you clear the ball, then your opponent trips over your and lands face-first on the ground, it's legal. If you clip your opponent's shin and then clear the ball, you're going to get at least a yellow card warning, and probably worse. Never slide tackle unless you're wearing form-fitting shin guards of the proper size. You're throwing your leg out there to be tripped over, potentially, or stepped on by a pair of cleats, making it essential that you guard yourself. Shin guards might not be the most comfortable things to play in, but if you're going to be taking it to the ground, they're absolutely essential.
Only go to the ground as a last-ditch defense. Run parallel to the player in possession. Watch the ball closely and wait until you see separation. Check your emotions at the door. Slide into the tackle, don't jump into the tackle. Make sure you get the ball. Protect yourself.