- I have this really cool stunt I want to pull, but I don't have quite enough movement to pull it off.
- I want to shoulder-check the death knight into the path of the lightning bolt trap we know is about to go off.
- I want to leap off the back of the House Orien lightning rail, jam this metal spike into the nightmare that's flying after us, and then let the chain attached to the spike fall to dangle in the lightning stones that power the rail, electroucting the nightmare.
- We have a whole crowd of innocent bystanders here. I want to to throw myself at the incoming Fireball "bead" to trigger it early.
In my 4e game, those are all in the realm of p 42: Aths or Acro for the movement (fall prone and take damage if fail), forced movement to bump the death knight, an improvised attack with the spike, Acro for the Fireball bead.
Examples I can think of from play are using a flask of wrestling oil to increase forced movement on a stone golem so it would slide into a wall of fire; using Thunderwave to blast a demon through the side of a timber house; using a statue of the Summer Queen to dispel a black dragon's sphere of darkness; etc.
In 4e, it requires an intuitive balancing of action economy and effects. In 5e, I'm not sure how to handle this stuff only because I don't have the same intuitions for action economy, damage and effects.
We set up this elaborate maneuver that culminates with the cleric casting a certain spell at a certain time, but the rogue's about to die. The cleric wants to utter a desperate plea to her god and try to break the "can't cast a spell except a cantrip in the same round you cast a spell with a bonus action" rule.
That one is tricker because it breaks the action economy.
That said, we do that in my game from time to time: when the
Vecna got inadvertantly pushed over the side of an earthmote, the paladin went after him "doing a Gandalf" and we allowed a free basic attack.
For the desperate cleric trick in our game, the players would probably try and weave a hp-boosting effect into the ritual (eg use Healing Torch or Battle Cry to power the ritual, with the healing side-effect helping the rogue).
Given all of the above, I doubt I'm as freeform as Rob Heinsoo. (Not that I feel any shame in being a weaker GM than him!)