The way slime blocks work in Minecraft is as follows:
-A player holding the jump key performs a normal jump on contact with a slime block.
-A player holding sneak does not bounce at all.
-And a player pressing neither of these two buttons will bounce to a height proportional to the falling velocity (thus usually higher than a player holding the jump key).
This is me holding the jump key.
And this is me not pressing the jump key.