what happens if i fire on coil which is free from one side ???
Depends on a few things...
If a coil leg is truly just hanging in the air... that's an "open", there's no completed circuit for the electrical energy to flow. Unlimited (infinite) resistance.
If that leg happens to touch something inside the atomizer, depending on where it touches... that's a "short", an unintended circuit of virtually no resistance between the negative and positive terminals of the battery.
On an adjustable power "regulated" mod, the circuitry protections should notice both "out of range" resistances and refuse to fire or pass current to the atomizer.
On a non-regulated (no circuitry) "mechanical" mod... the "open" should do nothing. There's no circuit being closed... no current flow.
The "short" situation however, is problematic.
As Baditude's pic above shows... it can be
very problematic.

No resistance, and no protections... the battery's output goes from zero to maximum immediately. They don't care for that much... at all.
Hence the reference "pipe bomb".
Always, always double check your connections... especially your coil tie-downs.

(And watch what the coil itself may be accidentally touching. Same "short" result.)
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Lol... got to rambling while watching the news. Sorry.