If the only cards on your opponent's side of the field are Defense Position monsters, this card can attack your opponent's Life Points directly.
Rulings
Individual Card FAQs
If your opponent has only Defense Position monsters on the field, you can attack directly with "Rocket Jumper", but if your opponent has a Spell or Trap Card, you cannot attack directly with "Rocket Jumper".
EARTH|
Level 3|[ Rock / Effect ]
ATK 1000|
DEF 800