This is a symptom of a worn drivetrain. When a cog (or chainring) gets worn enough, it can't carry the chain properly. So, if you pedal hard when the chain is on that cog, you may experience a sudden and disconcerting lurch in the pedal stroke accompanied by a strange popping sound. What's you're experiencing is the chain riding up and over the teeth on the cog and slamming back down again. Skipping can also be caused by a stiff link that binds when it reaches the derailleur or doesn't seat on the cog. Either way, it's something to have repaired ASAP.