To pre-empt 3♣ with 6 card suit? 

As Robson explains ( Andrew Robson Video) as you cannot do a weak 2♣ which in most systems is a strong bid. It is very disruptive as the opposition will not know whether to get ot 3 or 4 of a major. 

Disruption with a pre-empt is best when vulnerability is NV versus V and in positions 1 and 3.

After East bids 3♣ south is too weak to bid.

West could consider a raise to 4♣ (likely level of fit 7+3 - though it could be 6+3 Law of Total Tricks) though if North doubles they will get to game, that could make. a double by North could give South a difficult bid to 3 or game in a major.

North has a difficult choice 3NT with good stops and the most balanced of hands 4333 but he is too weak. So will probably double. As North has 15 HCPs, east 7 (4-9HCPs) so South is likely to have 9HCPs (40-15-7)/2=9 so game could be on.

East passes.

South has a difficult decision. To bid 3♥  or 4♥ . Either could be right.

On this hand 3 of a major is correct and 4 goes one off (or it should).

The question was raised whether 3 pre-empts with 6 card suits should be alerted. This is not an EBU requirement.

Some interesting links on pre-empts