Ill copy my explanation from another thread
Pendulum cards are cards that can be summoned as regular monsters, or casted as spells. That's why they have 2 effect boxes, upper one is the spell effect, and lower one your classic monster effect.
To activate a Pendulum as a spell, you place it face up in the all new Pendulum Zones, 2 zone located at the sides of the traditional board. Once played there, they stay there as continous spells, and can't be removed or replaced unless they are destroyed.
Cool thing about the new zones is that with them, Pendulums monsters don't take your 5 traditional spell zones.
when removed from the field Pendulum cards don't go to the graveyard, but to the extra deck(previously fusion deck)
Now for the true use of Pendulum cards. The Pendulum summons.
See the number in the red and blue arrows at the side of the card? that's its scale.
When you have both Pendulum zones occupied by 2 Pendulum cards, and they have different scales, you are allowed to summon as many monsters from your hand or Pendulum that were sent to the extra deck as possible as long as their levels are between the numbers in the scales
for example, if I have a Pendulum with a scale of 1 and the other with a scale of 9, I can instantly summon 3 blue eyes white dragons in my hand, since their level (8 each) is between 1 and 9 (eat your heart out Kaiba)
This can only be done once per turn, and the most powerful Pendulum cards usually place even more restrictions on it.
Hope I helped.
