As scrutiny of executive compensation intensifies, some companies are using a little-known technique that keeps potential payouts under the radar. Six Flags Entertainment Corp. and Tempur Sealy International Inc. have awarded millions of dollars in stock to top bosses and given the equity a unique value: zero. To use that figure, the companies set performance targets they said were unlikely to be met. Doing so is rare. Few boards set impossible goals and even the loftiest targets typically have some chance of being met. That’s been the case at Six Flags. Two of three improbable goals have been achieved, resulting in windfalls for executives. The third is still outstanding.