I'm trying something new (for me) and I'd like some opinions on if this is beneficial or just potentially harmful.

After my routine I'm usually at a 90-95% erection level, I guess I get subconsciously excited when I near the end. I've been finishing off by making an OK sign with my index finger and thumb at the far end of my shaft. I slide my hand down the shaft as far into the pubic bone/scrotum as I can get my hand and grasp with my other fingers for support (lube is slippery). As I hold this position my shaft becomes hard beyond belief within 30 seconds, along with the tip of my penis. I just hold it there and massage the far end of my shaft and tip because I get bored.

I've been doing this for about 25 minutes now, with 2 minute breaks so the blood can circulate. Has this been used as a regular exercise before? Is it dangerous? Is there a benefit in terms of speeding up gains, aiding in cementing gains, or enlarging the tip? Thanks in advance.