How does a backspin affect a shot?
A ball with backspin will bounce slightly more backwards when it
hits the ground, or slightly more forwards if it hits a
ceiling.
In basketball, this makes the ball more likely to fall through
the hoop if it hits the back of the rim, or more likely to bounce
high (and possibly fall into the hoop) if it hits the front.
In tennis, this makes the ball bounce higher on a normal shot,
making it slightly easier to hit. It can prevent a drop shot from
going as far, also, making the player run further in order to catch
it.
In golf, the ball will bounce a shorter distance (or possible
bounce backwards after a very high shot).