Can use on erect or flaccid. Start with a little rather than a lot, some of them are very effective and if you overapply you can end up with a dead stick so to speak, oh it might be hard but you won’t be able to tell. Also if you use too much it’s easy to get it on your hands your partner etc. . . And they’ll all be numb too.
Basically the one I have is a spray, couple sprays is all it takes , I keep it to the head and about half the shaft . It takes 10 minutes or so before the effect is noticeable. If you get the application right nothing feels numb or insensitive, I just don’t come to edge unless I’m trying. I think it’s good stuff but can go overboard and end up ruining things lol.