According to the website of one such product, the safe application of traction encourages tissue cells to divide and multiply, a process called cytokinesis. Over time and with great effort this will lead to tissue growth. The FDA considers these low-risk devices (Class 1) and so provides only general controls intended to be followed by manufacturers.
Herbal remedies, Kegels (pelvic floor exercises — learn more here), and exercises have all been mentioned as potential ways to make your penis bigger. However, we’ve searched and searched for medical evidence that these techniques will work to grow your penis, but there’s simply zero, zilch, nada, no evidence. You may find lots of unverified anecdotes, which come from a sample size of one, but they simply cannot be trusted. After all, no one on the internet knows you’re a dog (or liar). We recommend that you avoid these techniques.