When I was a young hockey player, there was a word that was commonly used in reference to how players would think the game which I haven’t heard as much recently. That word is "anticipation". Today we hear more about “read and react”. Wayne Gretzky was on top of the hockey world at that time (when I was a young player) and possibly the use of the word anticipation was driven in part by his style of play.
For Gretzky, it didn't seem like he was doing a lot of "reacting" so, "Read and react" seemed, well, not quite right. Instead it seemed like he was anticipating what would happen before it happened. Gretzky is so famous for this that Silicon Valley luminaries such as Steve Jobs have been caught quoting Gretzky saying that he tries to "skate to where the puck is going to be." They used this quote as a guide in their efforts to build new technology products that will arrive right when consumers are ready for them.
Why has "anticipation" receded into the background a bit in terms of how coaches nowadays describe what they want their players to be striving for? I don't know. But, this doesn't mean that players aren't doing it.
If a player has used a certain tactic in a certain situation and it has worked ten times in a row, that player will have expectations that it will work again the eleventh time. This is to say that they will anticipate it working. And, they should! But, this can be dangerous if this means they aren't ready to deal with the circumstances if it doesn't work. In particular, this is dangerous if the tactic not working also correlates with them being out of position.
This brings us back to slip unders. Defenders use their stick to control our options. Sometimes this means taking away access to certain areas of the ice and thus driving us into other areas of the ice. Defenders will take away access to scoring areas, which would obviously be valuable real estate for our team's offense. But, it can be extra dangerous to access that territory when defenders are using their sticks to try to take those areas away.
Because the defender is anticipating their stick positioning to work to take away that access. If you can break that tactic down and beat it, not only do you get access to ice they intended to take away, but you also, surprise them. They have to rethink their entire defensive posture after their tactic is ruined and that is to your advantage.
A magician setting up and then
Now, if one is Connor McDavid or Jack Hughes for example, defenders will not trust their tactics as much since they are known to be super elusive. Defenders will be more ready for those tactics to fail and won't be nearly as surprised when these guys beat stick-containment. But, guys like that can take even a tiny edge over a defender and turn it into a great scoring chance. So, in their case, even less-surprised defenders are in deep trouble.
Whether you have a reputation for elusiveness like McDavid and Hughes or not, work on slip unders to give you a tool to break stick-containment tactics that defenders use so you can put them on the back foot while you overwhelmingly take the initiative!