The offside line is critical in determining how much time and space is available. Who calls offside?
The place you cannot afford a mistake is centrally - in the danger and scoring zone.
One of the centre halves should determine the offside line.
To look at a game and decide if a team is playing an intelligent offside line.
That playing a high line is too risky.
Both time and space are affected by the use of offside and the tactical decision of how high up the pitch to begin defending - hence how much space is available.
Offside has been used as a highly organised trap to catch players off time and time again (the Arsenal back 4 of Adams, Bould, Winterburn, Dixon in the 1990s). With changes in the rules (level is onside, players coming back are onside) teams now use it as a way of moving the opposition away from their goal when they push forward. This is if you like an honest game, not relying on trapping players as an end in itself. The biggest single concern about offside is that if you play a line too far forward, and leave space in the danger zone and get it wrong, the opposition may score.
A goal in football is hard to get back. Therefore many factors have to be considered when deciding how far forward the offside line is.
Are you leaving an easy ball for the opposition into the danger zone? If you are, you are too far up.
The Pitch size, which is linked to the point above
Conditions (wind etc),
If the opposition has pace up front and so would like space in the danger zone to exploit,
How good your opposition are (see later). The better they are the deeper you defend.
How good/fast your keeper is as a sweeper.
If your covering player is being consistently seen and the space used by the opposition. The forwards are clever.
It is essential that the whole team reacts as the ball is played as effectively the opposition become onside. The team needs to drop off and cover team mates and space. If one of the defending team is sweeping/covering, and the opposition move in to the space, s/he simply steps up to catch them offside, dropping again when they are not looking.
If the opposition is successful in delivering balls in behind our defence to forwards, the line is too far up. It is better to be too far back than too far forward. It is also very dangerous to play offside if the opposition player with the ball has lots of time to wait and pick a pass. So if the ball is cleared into a wide position, or up to the opposition's back four, be careful how far you push up. In central areas where you can pressure the ball more easily and deny time, you can push up in more safely. When your own team has a corner the defence will be at the half way line. This leaves a lot of space in between them and the keeper (danger zone) . If the opposition's keeper can deliver a ball into this space then you could concede. The key point is be aware of the danger and drop back to deal with it. On the next practical drill you will see a drill that will emphasize the danger of defending too high up the pitch.
Where the defence draws the offside line is one of the hardest tactical decisions to make
Attackers with pace (most teams have one) love playing teams who push up too far and leave space for them to run into. They may get caught offside sometimes, but when they beat the trap they will score.