Signifying is related to the word "sign.

Christ's death was a sign (of the death He died at the foundation of the world):

Matthew 12:39 But he answered and said unto them, An evil and adulterous generation seeketh after a sign; and there shall no sign be given to it, but the sign of the prophet Jonas:

40 For as Jonas was three days and three nights in the whale's belly; so shall the Son of man be three days and three nights in the heart of the earth.

A death in demonstration indicates a sign. It signifies the reality.

God's elect have likewise entered into the condition of death in this Day of Judgment as God has turned the world into hell (the grave). Thereby we are demonstrating death whose reality was in Christ at the foundation of the world.