Punish game on marth off down throw on the 4 primary DIs you need to worry about:
- On DI away you pretty much always want to regrab. The primary exception is the case where if you regrabbed it would put you in a position where you can't followup (for example, under a platform where if he DIs onto it the followup isn't 100% guaranteed). In these cases dash attack is actually pretty good. Eventually you just fair him off this, but you can actually regrab full DI away for a very very long time: I'm talking like 80% on marth.
- On *most* slight DIs away, you can actually regrab, but I do believe there exists a slight DI that cannot be regrabbed. On this DI, at low percent, you should be able to take a step and uptilt. I'm not positive that this true combos, but he shouldn't have time to disrupt you meaningfully. You could try to ftilt in this situation, and just hope that he's in a bad enough situation afterwards that you can pseudo-combo or pressure him but I don't really like it... *shrug*. Once he gets to around even 15%, I would switch to SH upair, which is auto cancelable. You can actually fastfall for a couple frames right at the end of the SH upair and still get the auto cancel, and doing this perfectly puts you in a really good position. It won't generally true combo (though it can, but marth has very limited options to beat you uptilting or destroying him if he double jumps). One of the biggest issues with sheik combos off down throw that people don't realize is that the reaction is actually *very* difficult...I'd honestly compare it to tech chasing, once you get to the point of reacting to subtle differences in DI. For example, distinguishing between various slight DIs is rather difficult, so I honestly just dash SH upair all of them once he gets past the very low percents to remove ambiguity in my decision making (though purely optimal play might involve regrabbing certain slight DIs, even as his percent goes up a bit).
- ***LOOK AT THIS*** Until 27% after the throw, starting at 0 (roughly 0-16 before the throw), if they DI slight away with the 45 degree angle notch you can regrab without dashing. On even slighter DI away (closer to no DI, than away) you can't regrab, but the uptilt hits guaranteed. You can also upair quite easily on this DI with no dash required.
- On no DI you uptilt or upair; generally you want to delay the uptilt a little bit (though if you get predictable with your timing, it will be easy to SDI). The goal at low % is to get both hits of the uptilt, where it can lead to another uptilt, a regrab, a fair/nair, or an upair. FJ rising upair is super good around 20% (after the throw) onwards.
- DI behind you is the same, but you want to turn around before the uptilt to make it possible to get both hits of the uptilt. In any case where you uptilt you need to be aware of how likely it is for you to get both hits of it (especially given potential DI from your opponent). In cases where you don't get the second hit even at low/mid percent, it's still usually possible to combo if you IASA pretty much perfectly; the hard part here is that occasionally it is ambiguous whether or not the second hit will get them. If you're too busy waiting for the second hit to connect, you won't IASA early enough to convert the uptilt into a fair/upair. Luckily if the opponent SDIs the first hit to avoid the second, it is usually pretty reactable. Again, FJ upair is probably the best thing starting at around 20%.
I didn't really go into depth about it, but pretty much all your uptilts should lead to fair/upair, and your upairs should combo into fair/bair OR put him on platforms (where you do more bad things to him).
Just as a reference, this is a great example of how to combo the DI that should make sheik players go oh babyyyyyyyy when they see it:
In retrospect, uptilting while on the side platform instead of immediately fairing after the upair may have been optimal, but I probably was confident in the DJ read and wanted to keep him low when I faired to make it easier to cover all his recovery options (the higher up he is, the less useful fair is, since it doesn't send that far). It's probably debatable, but either way it's a great situation.
The last thing I think is super important off dthrow is killing him, which, again, is very difficult to do because of the reactions required. At 78% before the upsmash, tipper upsmash will kill on FD with optimal survival DI (which leads to my theory that FD might actually be better for sheik than people think). This means that if you grab him at 65, pummel twice, and dthrow you can kill him if he doesn't DI away on the throw. Here's how it works:
- DI Behind: Upsmash (you don't even need to turn around, though at higher % you might need to)
- No DI: Dash JC Upsmash
- Slight DI away: Dash JC upsmash
- DI away: regrab, walk ftilt, fair
Once you grab marth at this percent it's almost literally a guaranteed kill if you execute correctly. You can tipper upsmash everything as a true combo, except DI away. You can actually regrab the DI away at this very high %, and if he keeps DI'ing away you eventually get near the edge where you get dthrow-->fair into a guaranteed edgeguard. As a mixup with fair, you can walk into an ftilt and if they survival DI, you can upair/fair off the ftilt. I haven't practiced it much (beyond checking that it's a true combo in training mode) but as I said, the reaction is non trivial.
I would probably ideally talk about platforms somewhere in this post, but I'm not really sure what to say. You can often just waveland onto a platform and regrab everything, but if you don't have enough time he can tech in place-->spotdodge before you can grab. Something that every character can do that is fairly unexplored that would work for sheik is: 1)SH under platform 2) react to tech in place with an aerial or DJ aerial 3) react to tech roll with DJ waveland-->grab. This should be a perfectly reactionary coverage, but it's hard and people are lazy l0l. I think these are the only two variations of full option coverages that are really worth mentioning so I don't have much to say on it. I guess there's also the one where you can stay below the platform and AC fair the tech in place, land, then SH and instant aerial the tech roll. This should work, but requires a specific setup (you need to have already be at your peak and have your fair actively hitting vulnerable frames of tech in place so you can touch the ground asap) so I'm not exactly sure how often it is useful. Edit: Dthrow-->FJ-->autocancel fair on the tech in place/miss tech-->grab the tech roll is super good at higher %s. Dair also has its place in platform tech chasing, but it pretty much only covers all the options if you sort of land with them.