How to play Offside trap?

Been a long term user of the Classic tactics but decided to give the new way a try in FM2011 and it's been a breath of fresh air to be honest - having to be very careful at the mo due to the tactics bug mind - but it seems I can for the first time, create some really nifty new tactics and speak to my players as if they are there (as it were), but one area im unable to fathom out is how to play the offside trap. If Im playing Attacking, Controlled or Standard, my defensive line will be at the very least 50% (meaning they'r not deep) but there doesent seem to be an instruction for this. I could do it using the Advanced area but then when i switch from Balanced to Defensive for instance, it will still leave Offside ticked - is this the only way to use offside?

I think so. That's how I have it - just need to remember to untick it when you change mentality.