Originally Posted by Povl Carstensen
That is not possible to see exactly when filmed from behind.
No, you can impart momentum, thrust, power without being perpendicular. On the other hand being unperpendicular is a major factor in generating spin, both vertical and horisontal. Imo. And unperpendicularity to the target direction is nescessary to compensate for the incomming balls direction.
Edit: Sorry, I see this has since been discussed in http://tt.tennis-warehouse.com/showthread.php?t=450567
Yes we discussed the square vs slightly closed face in depth already