I've used them twice and they worked well both times although we never used maintain an untendeble position. If it's a situation like two gangs just stare at each other and bang their shield and suddenly someone decides it's massacre time they shouldn't really be used (unless both parties take cover after the first exchange and begin trying to kill each other in earnest).
Distance is important. Like the parties are aware of each other but don't see more than what their scouts say or they see each other far away and only sporadically like when someone is running between hills or burnt out cars or whatever. I'm not sure it's stated but the reason you might want to stay before 9:00 is you can move freely without having to roll or fight your way to get someplace. That is, there are still ways not in someones line of sight. If you have played Counter-strike imagine de_dust2, when passing by the middle doors the teams can take shots at each other but unless someone is lucky or has the fuck off big sniper nobody's going to die. Then there is some manuevering (and shooting people blindly through walls) but finally the terrorists have to try take a bomb site (siezing by force) and then a bunch of people always die.
Both of the battles that I MC'd had different things acting as fog of war. The first battle was fought at night so different gangs could move around protected by that. The second battle was an airship boarding the roof of a skyscraper and the attempt to sieze the top floor. So the roof guards took some shots at the approaching ship but backed off when it got closer. The incidental fire also came when some of the characters tried to assess the situation on the top floor by peering through a roof window.
Follow through on someone elses move example:
One player leads a gang and jumps down a floor through a window in the cieling and start shooting everywhere. I counted this as siezing by force. Another character was just somebody in the gang and also joined so that was obviously follow through on someone else move and I would have given her +1-harm if she had just shot but since she was keen on not killing anyone I described one enemy who was crouched behind a bookshelf was just setting his AK to full auto so she tackled the bookshelf and I ruled that as dominating someones position.
Covering fire:
The next tick they realized their enemies were both better armed and better armored than their gang. The gang leader was like "I kill them all!" which was obviously sieze by force and the other character was like "I take his AK and shoot over their heads" which sounds like covering fire to me. She succeeded so their gang didn't suffer any harm in the next exchange.
Maintaining an untendeble position secure sounds to me like it's a defensive move. And people, not even in apocalypse world, like being shot so I think it's like if you succeed you keep everyone the fuck away by having a clear shot at anyone who approaches, also you spend alot of lead so that people know not to peek into your trench or whatever. Failing there is a gap in your defense and enemies have the opportunity to shoot back at you since they have climbed your walls or stormed your trench or something like that.
Another thing that I think helped alot was having the countdown clock visible to everyone and explaining: you can do one thing per tick but after 9:00 doing anything is risk getting shot because the enemies will be all over. If you storm them right away it will go to 9:00 right away. If they feel like they need to organize their troops or defense or whatever they will likely use the first three ticks for that (have lots of civilian NPCs or berserker NPCs that they care about in the battle and you will see who cares for whom or not). And if someone storms off that's good too because either they are super bad ass and will win the day or they will get shot to hell.