The X is a good drill and you should buy the tape.
Here's how it works (and I'm not trying to spoil it for anyone, if you plan to buy the tape then maybe disregard this and just get it straight from the horse's mouth)
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You mark the table in order to find the exact way to place the balls, with diagonal lines running from the center of one corner to the center of the furthest opposite corner. Without very exact ball placement, the drill is less valuable.
Place balls as shown, start at the center of the table, and shoot one of the balls dead straight into the corner, and try to draw back to exactly the center of the table. You must do this with NO practice strokes, which is the tough part. You must address the pall carefully and perfectly and then stroke carefully and perfectly. If you make it back pretty near the center, repeat for the remaining 3 balls. If you get out of line to the point where you leave the 'box' or hit a rail or whatever, reset. The order you shoot them in makes a difference in how much recovery you're able to do once you get out of line... so maybe arrange them at random and shoot from lowest numbered to highest or whatever.
If you get all 4 and can get the cue ball back to the center, replace the 4 and try to do another set. If you can make it through 3 sets you're probably a champion.
2 things to focus on:
- hit dead center of the cue ball and dead center of the object ball, so that the CB goes straight back instead of veering a bit to either side
- carefully control the speed of the draw so that it's the same each time.
Remember - 0 practice strokes.