Ah, the 290. I currently have 5 sanctioned 290's, with never surpassing the front 9 in sanctioned play. I have had a tendency to finish out games rather strong this season. Although no 290's (yet!), I have had the back 10 for a 279, and have had the back 9 twice for 270, along with the front 9 twice!
I really don't think it's caused by anything except making an adjustment late, making better shots, and on occasion, catching a lucky break. My first 290 last season, I made an awful shot my first frame...pushing it out and nearly leaving the bucket (the 5-8 stood), before punching out. Later that year, after a bad 10th frame the previous game, I made a big move to start the final game, left a solid 4, then sheet.
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"Chicks dig the trip 4" -Randy Pederson