This is a common mistake that gets made even in high elo, which is where in a retake situation, the smokes player will place their smoke at a chokepoint instead of on the spike when they are trying to smoke defuse. It's a small mistake but very easily fixable by putting the smoke on the spike instead of the choke point where attackers are playing from.

Smoking the chokepoint gives the attackers smoke cover from where they can easily spam any defusers without much worry of getting shot back, especially if they have a phantom.

Smoking the spike does not give them this cover. If they try to spam the spike, they are still exposed to other angles from which either the defuser or their teammates can swing from to easily kill any attackers trying to spam the defuser.

If there are multiple chokepoints from which attackers are playing from (eg. hookah and b long on B site Bind) 1 smoke is required to hide the defuser whereas smoking the chokepoints would require multiple smokes or leave angles exposed.

If you just started a retake and are unsure of where spike is, it's better to smoke chokepoints to isolate angles.

If you have site control and can start defusing the spike, smoke the spike, not the chokepoint.

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TL;DR If you want to smoke defuse, smoke the spike, not the chokepoint.