The ability to read incoming spin and select the correct counter-spin response. This is the understanding that spin behaves predictably on contact with surfaces — and that matching spin types creates awkward contacts while opposing spin types creates clean ones.
First, read the incoming spin from the opponent's paddle path and ball flight. Backspin floats with a low bounce; topspin dips with a higher bounce. Then select the counter: against backspin, use a topspin stroke (low to high, brush up). Against topspin, use a slice stroke (open face, push forward). Alternative: against backspin, drive through the ball with enough acceleration to "bully" the spin off — aim higher than normal. Advanced: build compound spin by using opponent's backspin as fuel for dramatic topspin dip.
Using backspin to counter backspin forces the ball to completely change not only its direction but also its rotation — an extremely difficult contact. A ball coming toward you with backspin rotates the opposite way as a ball traveling away from you with backspin. Most players don't realize they're fighting physics when they slice against a slice return.
Instead of just countering backspin, elite players ADD their topspin to the opponent's existing backspin, creating dramatically more dip than either spin type alone. The opponent's spin becomes fuel for your shot — the more they slice, the more your topspin dips.