What is your frame?

I experiences this myself been pulling hair off frustrated why my old beater bike got skipping everytime I accelerated hard on red light. Its not single speed, its normal beater bike with gears. I looks at the chain, its good, cog also good, chainring good. I scratched my head so hard why this annoying phenomenal happens. Got LBS to check the bike for me because I really have no idea.

They ask me to show them what happens, I did the same and chain skipping. They told me you accelerating way too hard for your "frame". Your frame is flexing hard.

Its bike frame that cause this, its cheap no butting hi-ten frame I use for beater bike. Now, I carefully accelerating, no hard mashing or chain skipping happens.