In other words (saying thesame as BusBoy):
With the thrust levers in the CLB detent:
If you press the red buttons on the thrust levers you get climb thrust, as this is where the thrust levers are.
If the autothrust fails or you push the autothrust button on the MCP (which is a non-normal way of disconnecting it and is seen as failure by the system logic), you get thrust lock. This means that the current thrust is held, until you move the thrust levers. As soon as you do (even only a little bit) you get thrust commanded by the thrust lever position you set.
On the 737, if I remember correctly, one can set the N1 limit in the FMS, and then select N1 mode. The autothrottle then sets this N1. I may be wrong though, it's been awhile.