Hitting the ball immediately after it bounces, right off the short hop. It's basically a pick-up shot, and it's one of the hardest shots in tennis because your timing window is tiny. You're usually hitting a half-volley because you got caught in no-man's land between the baseline and the net. Federer made these look effortless. They are not.
"Caught at his feet in no-man's land, he half-volleys it back deep. Incredible touch under pressure."