My crank slips I think I've got a 2015 premium bmx. When I start to pedal, the left crank kinda slips, if that doesn't make sense, let me siimplify it. When I pedal, it's like it doesn't grab the chain or something, like it pedals and moves the chain but the wheel doesn't spin, only at first tho then it rides fine. But only when I try to pedal from the left side. I also have a freecoaster, maybe that's why but I don't know
My son and I have had direct experience with this (he is helping me write this).  I realize this thread is very old, but I put this here in case others are finding it.
First, check the chain.  If it is worn too much (they elongate over time) it won't grab the teeth.
Second, Check the chainring,  Are the teeth sharp?  If so, you need to replace it?
Third, check the teeth on the rear cog.  Again, if the are sharp, you need to replace it.