On the forum today, someone posted questions about the fifth-ball attack, and why players tend to miss the fifth ball when the third ball is against backspin. Specifically, he wrote, "I've noticed that the 5th ball is missed quite often when the 3rd ball attack is against under spin."
Some quick definitions:
The most basic third-ball attack is when the server serves backspin (usually short, at least at the higher levels so opponent can't loop it), the opponent pushes it back long, and the server loops, often looking to end the point on that shot. The most basic fifth-ball attack is when the server serves backspin (again, mostly short), the opponent pushes it back long, the server loops, the opponent blocks, and the server either smashes or loop kills.
The main difference between the third- and fifth-ball attack here is the back shoulder. (I wrote about this in a previous article.) When looping the backspin, the back shoulder drops; when smashing or looping the fifth ball block, the shoulder stays up. (It may drop slightly if looping against a block, but the key phrase is slightly.)
After lowering their back shoulder to lift the backspin, it's common for players to inadvertently lower their shoulder again for the next shot, leading to shots that go long. Plus the fifth ball (often a quick block) comes out faster than the third ball (usually a much slower push), and so the player is rushed, and a rushed shot against a quick incoming ball often goes long. (It rarely goes into the net since a player's first instinct is to hit over the net. When rushed, even dead blocks are often lifted too much and sent sailing off the end.)
The poster also wrote, "One coach I read said that you never attack hard against the 5th ball under these conditions (3rd ball was against under spin), that you must hit a controlled offensive shot and that the 5th ball is all about placement." While I understand the thinking behind this - placement is a priority, and consistency is almost always more important than creaming the ball (with creaming the ball consistently being high in the list of things top players learn to do), I would argue that in a fifth-ball attack, it is the third ball that should be the "controlled offensive shot" to set up the fifth ball. That's the whole purpose of the third-ball loop in a fifth-ball attack. While the server often does get weak pushes on the third ball that he can loop away for a winner (i.e. third-ball attack), more often he should focus on placement, depth, and spin to set up a weak return that he can put away on the fifth ball. (But note that placement is key to put-away shots - many players can return power shots if they go right where they are ready, usually the middle forehand or backhand areas, or too-obvious crosscourt shots. Put-away shots should go to wide angles or to the opponent's elbow, and down-the-line put-aways are often nearly unreturnable.)
This doesn't mean the server should always try to rip the ball on the fifth ball; only that the purpose of the third ball loop is to set up a shot that he can rip, and that if he does get a ball he can rip, he should (you guessed it) rip it, i.e. smash or loop kill. If he doesn't get a ball he can put away, then he should do another "controlled offensive shot" to set up the next ball, i.e. the seventh ball.
On the other hand, if a player goes for a putaway on the third ball - as many do, especially Chinese-trained loopers - then, if it comes back, it comes back so quickly that you should take a step back and loop the next ball for control. It really comes down to playing style and situation.