BC,
Implants are growth-promoting agents which - in cattle on an adequate to excellent plane of nutrition - improve ADG, feed efficiency and utilization, while promoting muscle(lean meat) development over fat deposition. Some are hormones or hormone-like in their chemical structure. Most have similar actions - promoting release of growth hormone from the pituitary; some may also have an effect on thyroid hormone and insulin levels in the implanted animal.
Probably the most widely used growth-promoting implant is Ralgro; it is zeranol, a non-steroidal estrogen agonist, derived from zearalenone, a mycotoxin produced by a fungus growing on corn. Zeranol - while NOT a steroid or hormone - has estrogen-like activity and appears to exert its effect by stimulating release of GH-releasing hormone from the hypothalamic region of the brain.
So...we remove the anabolic activity of testosterone produced in the testicles by castrating the bulls, and replace that anabolic activity with a subcutaneous implant releasing the specific anabolic agent over an extended period, to improve ADG, lean muscle growth, etc.
Much has been written about the safety of implants,with regard to consumption of meat from implanted animals - and it's often (incorrectly) incriminated as a cause of early puberty in girls, increased (recognition/diagnosis of) breast cancer, etc. But...analysis of meat from implanted steers contains many magnitudes less estrogenic activity than does meat from intact heifers, cull cows - and thousands of times less estrogenic activity than peas/beans, cabbage, etc.