Once you punch it up (I think that just allows a lot of pineapple juice to contact the meat), then you leave it in a tray in the fridge to pull in that juice.

If you intend on cooking it in the oven then you could leave the pineapple on for a bit, but it will probably dry out end get hard if left in all the while.

If you intend on braaing the meat then you definitely want to take the pineapple off and throw it away. If you want pineapple in the plate then cut another one.