>>> How do you get backspin on the ball? My son's coach told me that it is by swinging down on the ball and hitting the bottom half of the bat head with top half of the ball. <<<
Hi Bob
Since the bat-head is above the batter’s head at launch and can be at the knees at contact, it is obvious that it must have been swung downward. However, in all high level swings, the downward arc of the bat-head bottoms-out and is on an up-slope in the contact zone. As your post points out, far too many coaches believe (and teach) that the bat should still be angling downward at contact.
As far as “backspin” is concerned, a bat on an up-slope that strikes the ball below its’ centerline will also induce backspin. In other words, the bat does not have to be angling downward to produce backspin. – Below is video clip that shows the bat-heads path in the contact zone.
Across the plate view of the swing plane
Jack Mankin