What is “Pledge” in Magitech Chronicles RPG

Recently I acquired the Magitech Chronicles RPG book and I’m really confused what Pledge spellpool means, I searched all around the book and there is no clear explanation.

See, some spells have a cost, you use Lighting Bolt you pay 1 from your spellpool, other’s have a cost and that cost also gets "pledged". Does that mean that while the spell is going I can’t recover that amount of spellpool? Anyone knows?