Flagpoling requires you to go the US side and notify US CBP you are flagpoling, They will then give you an adminstrative entry refusal and then you return to the Canada side for processing.
Be aware that flagpoling at crossings in southern Ontario only accepted Tues/Weds/Thrurs and even then can be restricted/refused if there is a long line of people flagpoling. No idea on whether US check fingerprints from flagpolers but nothing is surprising with the US and not a big deal anyway if nothing to hide.
