Question about the newly spoiled card Hypothesizzle (Instant. 3UR. Draw two cards, then you may discard a nonland card. WHEN you do, ~ deals 4 damage to any target). Am I right in assuming that this creates a triggered ability upon resolution, which goes on the stack after Hypothesizzle has finished resolving? If yes, do you have any idea why it is templates like this and not with „if you do“?
Yes, this is correct.
Hypothesizzle has what’s called a “reflexive trigger”, which is new technology as of Amonkhet. Reflexive triggers haven’t shown up on a lot of cards yet, so most players don’t yet recognize them.
Reflexive triggers exist to allow their controllers to more precisely control how their spells and abilities resolve by not forcing a selection of targets until the reflexive trigger is put on the stack.
In the case of Hypothesizzle, you don’t choose a target for its reflexive trigger until you discard a card. While the exact nature of the discarded card doesn’t matter (beyond being nonland), the two cards drawn from the first half of the spell might change what you want to target. For example, if the card you discard is a Fiery Temper, you might choose to then target a 7-toughness creature rather than a 4-toughness creature.
If Hypothesizzle had the “if you do” wording rather than the “when you do” wording, the target for the damage would have to be chosen when Hypothesizzle was cast, before any cards were drawn from it.