When Yanmega tries to attack, do you evaluate the number of cards in your hand for the "Insight" Poke-BODY before or after discarding a card for Giratina Lv.X's "Invisible Tentacles" Poke-BODY?
The Insight Poke-BODY is evaluated before your attack even begins; therefore it is checked before discarding any cards for Invisible Tentacles.
Source: PUI Rules Team (2010-11-11)

Relevant cards

Note: All applicable cards may not be listed below.
HGSS4_98