How to pickwhip and position bulge center in After Effects to skinny legs?

1. create a null layer
2. then use motion tracker track 1 point
3. apply X and Y to null layer
4. Create new BULGE effect in a new Adjustment Layer
5. alt+click the Bulge Center timer icon
6. in the expression area, select the pickwhip icon, and select the Null’s position, that’s it!

Reference:

Help getting distort->bulge to follow motion tracking? from AfterEffects