Let ) be a symplectic manifold and be a symplectic potential.
Since is non-degenerate, then there is a unique vector field such that
If is a symplectomorphism that preserves (that is, ) then commutes with the one parameter group of diffeomorphisms generated by (the flow of at time ).
Written another way
Proof
It suffices to show that
We can differentiate at which gives
Where the last line follows since
since is the unique vector such that then .
Example
For a smooth manifold and cotangent bundle , when is the tautological 1-form, then the unique vector field from above is