Can someone explain what is the rule of hurdling on kicks?

[TigManinSC]

I have watched a number of games and cannot figure out what is legal / illegal when it comes to jumping over the line and blocking a FG/XP.

Here is the SMU guy jumping over the Duke OL while a teammate #81 pushes the OL down.

Was this legal? Exactly what is legal or illegal on these plays? It all seems very confusing and arbitrary.

 

[trich24]

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[Clemsonu90]

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[Tigerman91]

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[1Clemson]

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