Normal Summon "Steelswarm Mantis" by tributing this card to swarm the field by Special Summoning a "Steelswarm" monster from your Deck by this card's effect and another one from the Graveyard by the effect of "Mantis".
If you control "Steelswarm Gatekeeper" at the moment you Tribute Summoned "Mantis", you will gain an additional Normal Summon and perhaps perform another Tribute Summon, Tribute this card and Special Summon even more monsters.