What is a frozen pension plan and what actually happens when the pension plan is frozen and say the company makes a one time payment to fully fund it?
For a plan that is unfrozen, members stll working for the firm (actives) can accrue additional pension benefits by working there longer (for example on retirement the pension payment may be calculated by multiplying final salary by (number of years worked)/60.
Freezing (or closing to accrual) the Plan means the member can no longer accrue additional benefits by working at the company for a longer time period.
Any help?
I believe that freezing the plan is different than closing it to new entrants
freezing plan = no need for equity anymore.
Good question. I’d appreciate some nice answer as well 
No need for equities for sure.
Open: Any new employees and their pension benefits can be accural.
Closed: Only employee who were already in the plan can accural benefits. New employees are unable to join.
Frozen: No new accural benefit will be credited to the plan.