we connected in toronto in december 2015, and another trip in february 2016, both with westjet, and experienced this both times.
basically, when you connect from a canadian origin to a us destination via pearson, you will clear us customs in toronto. you no longer have to handle your checked baggage for the connection and customs, it is processed behind the scenes.
there are multiple points where your information is scanned and you move forward to the next step. right before you enter the "customs hall" is one of these. if your bags have processed through you are allowed to enter the customs hall and you get in line and clear customs.
if your bags have not yet processed, you are told to go sit down in the area to the right (where are chairs) and wait until your name shows up on th screens they have hung. the screens also show the flights for apssengers being processed, and the status of the luggage of that flight, so you have an idea of where in the process your bags likely are. also in that "room" are desks for the airlines who can help locate wayward luggage (see below). both times i have had to wait there an airline employee has approached me, scanned my boarding passes and determined where the bags were and let me know how long the wait was likel to be based on that.
One of the trips i only had to wait about 5 or 10 minutes. the other, however, my bags decided to take a little trip and stay in canada, somehow having been directed onto the domestic belts instead of the transborder (as explained to me later by westjet and delta). It took a good half hour for them to FIND my bags, but once they were found one of the westjet workers went into the back, retrieved them and put them into the transborder stream. a short wait later and my name came up on the board and i was allowed to enter the customs hall and clear customs. both westjet and delta did a great job of talking with me and keeping me informed as i waited, and also explained what would happen if the bags kept on their little adventure and could not be found in a reasonable time [after a certain time i would be allowed to enter customs so i would not miss my plane, and the bags would catch up later and be delivered to me by the airline - i was very grateful i had a four hour connection !!!].
most passengers can go right into customs, it is only a small number who get to wait for their bags to process through. and even rarer for bags to be like mine and want to stay in canada.
SW