Here is my take on phalanx warfare.
It starts with an initial push, try to break the other line from the very beginning. If that fails, troops fall back, dress the line and prepare for a surge.
Now ancient battles and formations were probably a form of crowd management, try to reign in natural instincts and try to avoid the usual problems when people are under stress and start to do crazy things.
They would have fought in surges because you don't want to exhaust yourself, most people can only do intense activity for a few minutes before they are exhausted. So there must have been a nominal gap between the two armies, just out of range of the enemy spears to give troops a chance to take their breath and start again.
"Oh, but people were like super buff back then !!!"
No, they weren't think more like "Third World used to back breaking labour tough" and none of them are seven feet tall mountains of muscle. They didn't have the proper nutrition nor the time to develop huge muscles, they had dietary problems and various diseases that affected them.
Most battles lasted anything from 30-45 minutes up to an hour or two which completely undermines the idea that they were pushing or fighting all that time. It was an exercise in trying to balance the exhaustion level of your own men, keeping a coherent formation and preventing the rear ranks from slowly pushing the front ranks forward so the gap that allows you fall back and rest narrows over time until you're nose to nose with the enemy.
I'm with Goldsworthy in that the Othismos is probably a figure of speech of the two armies locked into some kind of close fighting resembling men locked into a wrestling match, not some weird Rugby-style scrum. Such activities are only held for a few minutes at the very best since they can be extremely dangerous and the risk of a crowd panic and the men in the front line being crushed is quite real. Besides what's the deal in giving guys who are probably too busy doing anything but pushing a spear ? It doesn't make sense to have a pushing match and then give soldiers the most unwieldy weapon short of a two-handed axe in a close-quarters shoving match where the odds that one slip will have you pinned and crushed by the weight of up to fifteen men pushing on you in both directions.
And here's the problem, you're PUSHING, you're not fighting … How come they have armour and spears and all this crap if all they are going to do is a pushing match ????
Besides, wouldn't the fight go to the first guy who thought about bringing a knife to a pushing fight ???
I'm not saying that there wasn't a big scrum at some point, but they tried to avoid it, postpone it until the end of the battle as a last attempt to break the other guys when all control over the crowd is lost.
If battles are fought with surges and giving time to men to recover and surge again while trying to keep the crowd effect under control, imagine the impact of Celtic barbarians charging such a defensive line ? Suddenly troops are fighting men who don't pause to lock shields for a while and trade stabs with spears or swords, they keep on coming, hacking at everything that moves. That's really unnerving to even brave, well trained Hoplites used to battles going a certain way at a certain pace, they will feel the urge to fall back to gain some breathing space but the enemy keeps on coming they don't pause like civilized soldiers and that's when the Celts break Greek and Roman phalanxes.
And the most unlikely film that might help to illustrate this is Hercules, where troops break up a close formation, panic sets in and the battle is lost (probably not exactly the way it was done, but it might give a hint of how it might have happened)
YouTube link
And it also helps to explain the Macedonian model. They simply use spears so long and so dense that no Phalanx can come into contact and are stuck trying to fight an enemy that is pretty much safe from attack and can start to push you over if they move forward.
And it explains Roman formations, they probably understood that surging forward, stab a few times and then fall back with a spear while trying to remain in a coherent formation is extremely difficult to manage, so they switch to swords and throwing weapons to break up the line and develop a system where cohesion and keeping a shieldwall with your buddies is not that critical and they can keep up pressure by switching ranks and bring fresh troops, avoiding the crowd bunching problem. Romans make their formation both deadlier in that it can keep up pressure much longer than a phalanx, but it also has much greater depth thanks to their system of usually three, but up to five lines at times where the soldiers become progressively tougher veterans and your phalanx is being eroded until it collapses.
Phalanxes were extremely powerful when used against Asian armies who used a formation system, but put much more emphasis on missile fire to soften up the enemy and then force a weakened enemy to run with a well timed advance with spears forward.
Against such formations a Phalanx is like a battering ram, they have much heavier gear and can survive the initial missile fire and once they get into close combat range the momentum is enough to break a line of lightly armoured (if at all) troops with wicker shields and spears. That's how the beat the Persians because their formations were not designed to go toe to toe with heavily armoured infantry fighting in tight formation and why they liked to hire Greek mercenaries so much to supplement their own armies.