Any tackle is an automatic Down. When Downed you're completely vulnerable, and then when you get back up you're invincible for a few seconds. Most (all?) frames can chain multiple tackles, but most of them, the tackle is slower (but invincible, and in more interesting patterns than "dash directly towards enemy") so there's better things to do.
For all I know there might have been better things to do than chain Javelin tackle, too – certainly some weapons do a lot of damage up close, and would be decent competition – but this was silly. I think the Javelin and the Juggler should maybe have switched names, just for this.