TT18-A Launch Stability Enhancer
The TT18-A Launch Stability Enhancer is a stability clamp system designed to hold the ship at the launch pad before lift-off. It can be very helpful if -for whatever reason- the spacecraft can't stand on it's main engines or if it's out of balance and would otherwise fall.
When using the TT18-A keep in mind that it must be decoupled before you engage the engines to avoid a grand catastrophic failure of the tear apart kind. A simple solution is to put them into the same stage with the main engines, this way they will release the same moment the engines engage, resulting in a satisfying "clunk" sound as your ascent begins.
It is worthy of note that as of version 0.17. the staging sequence lists the TT18-A as a strut item and not a decoupler.