There are some mods with varied animations, tied to a key (like X or Q). Phantasy Calradia comes to mind. Please don't delay the mod over it, though. Maybe in an update.
As for the use of the spear, I perhaps caused some confusion by not illuminating more about the shield part. I'm not at all incorrect, having done years of research on this very subject - and that might be where I err, in skipping ahead to a main point without proper explanation.
Shields largely decide the technique. Actually they totally decide it, tbh. Pikemen used much smaller shields, so they could make a pike wall. Defense came from distance, vice coverage. Hoplites used a much larger shield, designed to overlap one another and "lock", thus creating a shield wall.
Pike Walls used spears of up to 16 feet in length. Shield walls used 8 foot spears, pointed at both ends to allow the stabbing of downed soldiers using the butt of the weapon. The butt was also weighted to give more momentum (thus more energy transfered through the spike), and to aid in balancing the weapon.
There is no under-hand technique in a shield wall. It simply doesn't exist, as one cannot pass a spear through a shield. There is a rank command to allow for a strike to occur After a wall is opened, left arm extends, and then an under hand stab - but that's no longer a shield wall. That's the only way it can happen in a formation.
The pictures on the vases are nice, but are not relevant. That's a single soldier on a battlefield who, since is out of formation, can use any style he wants. There exist many, many sources for information on all of this, even more so now in this age of interwebs than I had available when I was studying all this 40 years ago. Much information, I encourage folks to read the greats and learn.