Forwards are meant to score. Sometimes it is that simple. Sure there's so much other stuff that they can do, but in the end they are the scorers. In order for top forwards to accomplish this goal they need space to receive the ball and then space enough to shoot it effectively. In these parts we've called that part of the game tactical awareness. But how a forward gets space, what spaces they get and how they use it is unique to each player. Some use great on-ball foot skills, others are speed demons, some use size & strength. In an ideal world a player could do all of these. They'd also be in a league where they would get paid tons of money. In MLS players tend to specialize in one or maybe two of these.
Sigi Schmid's latest forward rotation with the Seattle Sounders has a player whose skill is dominant in one of those tools. There's MVP candidate Fredy Montero who works the ball at his feet to create space, routinely dribbling around or through defenders or uses his hips to swing the ball over to his open side. Montero is a perfect example of a forward that uses technical skills to earn his scoring opportunities. Over the last few games Sammy Ochoa has used his size and strength to win balls in traffic. He will either rise up to put a header on goal or body up to win the space to get the ball at his feet. Seattle's greatest speed threat these days is Mike Fucito. He can run down the direct ball, or force a high line into retreat because no defenders can keep up with him while he's on the move.
All have other attributes in their games, but at their core it is this simple. Each having a different key skill allows Sigi Schmid to use his rotation at forward to change the tactics of his side and forcing a shift by the opposition. This isn't the simple like-for-like that just uses a fresh set of lungs to take advantage of exhausted defenders, but shifts in which spaces are used by who and how.
When Fucito and Montero are paired up top Fucito tries to force the line back. He stays as high as he can to create space for Montero, but also it means that when he gets a through-ball or ball over the top he is nearly on goal. He will tend to do this from a space to the left of center, while Montero operates in the central channels, with some drift to the right or left. Ochoa will also be high, but mainly in middle or very slightly to the right. While he creates the same vertical spaces for Fredy it shifts Montero's main lateral space more to the left.
So not only can Sigi use his first forward sub like everyone and take advantage of tired defenses, he forces them into situations that they have not yet been in earlier in the match. In a zonal defensive system the right centerback may start the game cheating up in order to close on Montero because his partner in defense is back on Ochoa. That RCB has to focus on the skill set that Montero has and can recognize that Fredy isn't going to blaze by him without the ball, but maybe with it. He needs to mark a bit tighter.
Come minute 60 or so things shift. Ochoa comes off and Fucito on. Now a tired man faces a fresh and fast one. One who can't be marked as tightly because if he gets loose he won't be stopped. The LCB also gets that brief moment where he knows he won't be bruised as much. And then he realizes he'll need to cover more ground to stop the danger of Montero.
These shifts are true no matter who starts between Fucito and Ochoa. If the CB is already exhausted from chasing long balls to Fucito they don't get a break by then getting to face Montero, or by switching over to the strength of Ochoa. Sigi's triple threat at forward expands the space used and his tactical options. Even without dropping one into the midfield as he had done in the past. The Fucito/Ochoa pairing hasn't appeared yet, but that too gives him options. Like when Mike and Nate Jaqua had paired it pulls the two CBs tighter which then opens space for the cutting wide players.
Some teams may prefer that they replace their fast forward with a less good fast forward, but by using each specialist in different manners that creates more opportunities to score for every one forcing an even more reactive defensive posture by the opposition.