A vector field X on a Riemannian manifold $(M,g)$ is 2-Killing if $L_XL_Xg=0$. This vector field is a generalization of Killing vector fields. There are two articles in the literature review considering the geometric interpretation of this concept.
1. http://www.emis.de/journals/BJGA/v13n1/B13-1-OP.pdf
2. http://www.worldscientific.com/doi/abs/10.1142/S0129167X15500652
Now, I want to enlarge the class of conformal vector fields in the same line.
A vector field X is 2-conformal on a Riemannian manifold $(M,g)$ if
$L_XL_Xg=\rho g$
My question is: What is the geometric interpretation of this vector field? Also, Does the left hand side mean double dragging of the metric, or some thing else?