To simplify things, Sentinels are minions that take the form of vanilla 3/3s. Their sentinel effects trigger upon a certain action being fulfilled (General attack, Minion summoned, or Spell cast). Once their effects are triggered, they take on their true form.
Odd interactions happen with sentinel minions because their effects go off before the actions that triggered them resolve. For example, a Phoinex Fire destroys a Mind Cage Oni. The minion is destroyed, but it still puts a Phoinex Fire in your action bar because the effect triggers AFTER your opponent casts Phoinex Fire, but BEFORE the damage is dealt. A general that attacks a disguised Drake Dowager will receive 1 point of damage instead of 3 because the minion transforms AFTER you declare your attack, but BEFORE the attack is dealt.
In the case of Lavaslasher vs…let’s say Hundred-Hands Rakushi. You select where to summon Lavaslasher first. Then its Opening Gambit triggers because Opening Gambit occurs before being declared as “summoned”. Then Hundred-Hands Rakushi’s effect triggers, summoning the 3/4 body while weakening the Lavaslasher’s health by 2. Lastly, Lavaslasher is considered “summoned”, and will be a 4/4.
They’re still relatively new, and shrouded in mystery. I think Chromatic Cold actually doesn’t trigger spell sentinels for some reason, but I’m not sure.