im guessing your actor is a main actor, and you want to create some sort of slowmotion physics effect, you can do that by checking platform behavior i think, and platform behavior has its own gravity system, which is kinda better then the actual physics, just sometimes fails at collisions as any physics in c2 at the moment, and passes through objects if the forces are to high, anyway
so what you could do for a slowmotion effect for jump is when platform is jumping, set gravity force(original is 9.8 i think) set it to 2.5 then after 2/3 seconds using wait random(2,3) set gravity force back to normal that will make your character slowmotion gravity pull, however other objects will have the same speed, since your only doing this to 1 char, but you could use also timescale to lower the timespeed of gameplay i have something similar using in a game template il upload in scirra soon, its doing the same effect, as earn to die gamepplay, i hope we talking about same thing!
for rotating the object 20 pixels, and make it smooth, you have to use the rotate toward angle function 1 pixel and angle you want to turn, that will make your char if its in original position which is 0 angle if you want it to spin clockwise, (right ) youl have to say rotate toward angle 20 by 1 pixel now you can do this in so many ways, just depends which suits your best interests, there are a few how do i posts if you search them will answer your questions, this main thing is basically a regular question here!:)