A CRJ50 Captain discussed the confusion resulting from not understanding the meaning of the new Canadian descent clearance phraseology; 'Descend when you're ready to XXXXX.' He understood constraints were removed; but in fact they were not.

Narrative

Before the trip I had read the new rules regarding altitudes on Canadian STAR's ALPA had provided and thought I understood them. We were on our way to CYYZ; completely normal flight level at FL190 and we were on the WATERLOO FOUR Arrival given to us on our initial clearance. ATC issued an instruction to; 'Descend when you're ready to 12;000 FT.' We read back the instruction; set and confirmed the altitude pre selector. I interpreted it to mean 'Descend at pilot's discretion' which in the U.S. would void a crossing restriction and I began a very gradual descent. A few minutes later we received another instruction; 'Descend when you're ready to 11;000 FT.' Again we read back the instruction set and confirmed the altitude pre selector; I had the same interpretation and continued the descent. About the time we were coming up on ROKTO; ATC queried us about our altitude. Thinking this Controller wasn't aware of the 'pilot discretion' instruction we had received; I jumped on the radio to explain our previous clearance as to our reason why we weren't at 14;000 FT for ROKTO. ATC informed us that it's a common mistake especially given that the new rule only went into effect Thursday. I asked if he wanted us to climb back to 14;000 FT and was reassigned 11;000 FT as my altitude. I didn't have the impression that the Controller was going to write this incident up and he gave no instruction to us or any other aircraft to change altitude or heading to avoid a conflict. The flight continued normally to CYYZ. It should be noted that we never received a 'descend via' clearance; the crossings were expected to be followed by the pilots which is what I understood to be the case after reading the memo. What confused me was what instruction(s) given by ATC will void a crossing restriction. This event is very frustrating to me because it wasn't because a lack of preparation or an error of complacency; rather its source was a misunderstanding and at this point confusion on my part. Specifically; is there any clearance given by Canadian ATC that will void a crossing restriction? I don't have the memo in front of me to review as I type this; but perhaps that would be a better way to instruct it; i.e. always assume the crossing unless you receive 'xyz' instruction. If the memo said that; my apologies I simply forgot. It must be said that the new Canadian rules are a safety concern as they rely on assumption that the pilot will comply rather than direct communication.