You can cannibalize your body's resources to provide additional Source Points if your SP pool would go below zero as a result of channeling a weave.
Minor rank in at least one elemental Power.
Normally a weave cannot be cast if the SP cost of the weave exceeds the SP remaining in the channeler's pool. With the Overdraw feat hit points can be converted to source points at a 1:1 ratio. The overdraw cannot be used to generate more source points than the current character hit points.
Fytric wants to cast Prismatic Spray. The normal source point cost for this spell is 46, but her SP pool is down to 30. Her current hit points are 45, and she chooses to cannibalize 16 hit points to provide the SP necessary to cast the spray.
She ends her turn at 29 hit points, having successfully cast Prismatic Spray.