I seen a syringe and rubber tube full of it used before. Guy just holds the little tube under the knob.
Did a B/G/B once and the other dude did that, fucken ended up hitting me in the stomach. I didn't know and thought it was real, I fuckin freaked.

Then they pulled out the syringe to show me it wasn't real and bitched at me for messing up the take for his "Pop Shot". Needless to say I didn't feel bad about it.