This is a good question. It might be best answer this question in an outline format.
1. Hoplites from each army charged each other in order to create a break in the line. This collision was essential and important.
2. In some cases the outcome of a hoplite battle was decided right here as the collision would create a break in line. This would make some flee and this could lead to a easy defeat. In some instance, some armies might get afraid a go into the battle in a fearful way; this, too, would be disastrous.
3. As you can imagine holding the line was of utmost importance.
4. After the initial shock came the "push," which in Greek is called: othismos.
5. During this "push" the soldier would hack at each other in the hopes for victory.
Victor Davis Hanson describes the Battle of Delium, a hoplite battle of the Peloponnesian War, in A War Like No Others.
If you find Delium in the index, you can find his description of the battle.
Hanson has studied the subject, hand he has written other books with the same information.