I always tell the keepers and players I train with the most difficult shot for a goalkeeper will always be under his/her's hip's. Anything at a low height requires that the keeper work for the shot by either shuffling behind it or getting the body down under the shot.
As you can imagine there are several different forms of these low shots and how you can deal with them. So lets go through them from direct and simple to requiring the most footwork and energy to make the save.
first up is when the shot comes straight to the keeper's frame and they need to hold and smother in front of oncoming strikers, here we use the forward dive and smother which is as simple as doing a wide squat with your elbows together and knuckles touching the ground. Make sure to keep your head and chest locked over the ball once you feel your hands grab onto the shot.
and for when the shots are low and strong but require some footwork the moving forward smother is just as simple as it's predecessor the only difference the keeper steps into the shot while shuffling and pushes out the collection to either side.
When you don't think you can get your feet moving quickly enough to get your chest behind the ball the next best bet for those low sneaking shots that are just out of your range is the collapse dive. The collapse dive is a simple technique that every single keeper uses and has used. Just go from a keeper ready position, take your power step towards the ball and collapse over the side of your knee to fall down with style and make an easy collapse dive.
The drop step is a bit more advanced but it is needed for those last second low shot saves, most of the times these shots happen in 1v1s but can come from far out if the keeper's vision is being blocked. The drop leg is exactly what it's namesake says lifting and dropping your leg as a weight to force your body down as fast as possible. Don't expect to hold onto these shots make sure to get up quickly to get the loose ball.
Finally the full stretch low dive is necessary to reach out and make that huge low dive save, the most important part of this save is setting up your lunge and staying low! do not dive in an arc as to say don't go up on your dive just to come back down that will just give the ball more time to slip under your hands.
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