BitLocker uses a TPM (Trusted Platform Module) chip, built in to the motherboard, to encrypt the drive. You can not unlock the drive for use until you enter the password at boot up. Either you know the password, or you don't. If someone else has the password then I can only assume that it's their drive, and therefore any bypassing would be an illegal intrusion into one's system.
Other methods, if you don' have a TPM chip built into your motherboard, would be to use a flash drive to act as a TPM chip and hold the password, making it impossible to use the hard drive without inserting the flash drive into the system.