Channel's special ability is not available while paying costs

The special ability granted by Channel can be activated any time you have priority, but it won't let you do so *during* the cast of a spell. This is a problem, because you might want to cast a spell while holding priority, and if you activate channel before casting your spell, the life loss could trigger other effects. Channel should be treated as a mana ability.

Channel ability on an empty stack

Channel still active, but no way to use it while paying for a spell