In some behemoth fights, it is very helpful if you can interrupt or "boop" a behemoth. That will knock down the behemoth and leave it sprawled out on the ground for several seconds, giving the entire party a chance to safely hack away at it. Some behemoths have an unstable attack (or occasionally two) that can be interrupted by damaging the behemoth with a "heavy" attack of your own during its unstable attack.
The problem with this is that some attacks count as "heavy" attacks and some simply don't. If you hit a behemoth with an attack that is not considered heavy, that won't interrupt it. When you're trying to interrupt the behemoth just before it hits you (e.g., Embermane), failing to interrupt it means that it hits you. So you really need to know which of your attacks can interrupt it and which cannot.
The problem is that there isn't that much good information out there about which attacks will interrupt and which will not. Much of what is out there is contradictory, and it appears that various patches have changed whether particular attacks count as heavy. Some information out there is thus obsolete, which explains why it is so often contradictory.
My solution to this was to say, this is an empirical question, so let's go try it. Skarn is by far the easiest behemoth to interrupt, as when it stands up, you have a few seconds. It stands still, leaving no doubt as to whether you hit it or not. An interrupt will knock it over backwards. If it falls forward, you didn't interrupt it. Whether or not you actually landed an interrupt is pretty obvious for all behemoths, but some move so fast that it's not always clear whether you landed the attack that you were actually trying to land in order to interrupt a behemoth. These were solo hunts, so when a behemoth was interrupted, there was no ambiguity as to whether I interrupted it or someone else did.
I generally tested any attacks that were easy to test. That varies a little by weapon, but it typically includes left click, right click, roll+left click, roll+right click, and the Q special. The roll+click attacks are done by rolling and then pressing an attack button during the roll, which for most weapons, causes the player to exit the roll with a different attack from the one that is normally first in the combo for a simple click. As weapons commonly have several Q specials, and you can only load one for a given hunt, I generally only tested one such special. In some cases, it was the one I liked best. In others, it was whatever was easiest to test. I'll give details for each weapon below.
If a simple attack didn't interrupt a behemoth, I'd try pressing the same button several times in a row. Combos commonly make it so that clicking the same button repeatedly cycles through several attacks. I was surprised to see that in several cases, the first attack in the chain did not interrupt the behemoth, but the second one did. When I observed this behavior, I tested it repeatedly to verify that this really is what happens. If the first attack in a combo will interrupt by itself, I didn't test to see whether later attacks would also interrupt. If the first attack does, then you don't need an interrupt from later ones.
When the first attack in a chain didn't interrupt but the second did, I went to Embermane to test that same attack. In all cases, it turned out that the first attack in the chain could interrupt Embermane, but could not interrupt Skarn. That means that we have the weird situation that which attacks count as "heavy" to interrupt actually varies by behemoth. Regardless, the results below are generally for Skarn unless stated otherwise.
All testing was done without getting weighted strikes up to +6, which would allow some additional attacks to also interrupt. I don't have an easy way to get weighted strikes to +6.