How do i make custom swim thingy

[summer bramble]

Hello, does anybody knows how to make it so when player is inside a special part, he goes down slowly while inside the part, and jumping will make him go up (making invis terrain water wont help, whole point of not using it is to prevent lags for mobile players)

[narrow kettle]

Uhhh

Could use a spatial query to detect whose in the part

Then mess with gravity for the other part

[summer bramble]

ty