Here are the rules for B.A.S.S.
http://www.bassmaster.com/sites/default/...es_pro_am_1.pdf5.SAFETY:
(i) Safe boat conduct must be observed at all times by competitors. During the
official practice and competition, each competitor must wear a Coast Guardapproved
chest-type life preserver anytime the combustion engine is in
operation.
This preserver must be strapped, snapped or zippered securely, and
maintained in that condition until the combustion engine is shut off. Not saying Berkeley's rules are the same but, read what I highlighted in red. Notice that it states until the combustion engine is shut off. Not taken out of gear. Common sense would tell me if the jacket must remain secured until the engine is shut off it must be secure before the engine is started, too. Interpret however you choose.
Now here is Berkeley's same rule.
Safety: Safe boating must be observed at all times. Each contestant is required to wear a fastened (fully zipped and/or all clips fastened), U.S. Coast Guard approved, personal flotation device anytime the combustion engine is in operation during official tournament hours. All boats must be equipped with an emergency ignition shut-off device, which must be securely attached to the driver's body any time the combustion engine is in operation.
The driver must be seated in the driver's seat anytime the combustion engine is engaged in gear. Notice it states the jacket must be fully fastened while the combustion engine is in operation. It goes further to state that the driver must be seated before the combustion engine is in gear. That right there tells me that Basschamps intend for your life jacket to be strapped on before you start the engine.