Explanation

Box 1: BEGIN TRANSACTION
Box 2: UPDATE ... #Disktable
Box 3: IF... SELECT INTO ...#Disktable
Box 4: .. DELAYED_DURABILITY = ON
The COMMIT syntax is extended so you can force delayed transaction durability. If DELAYED_DURABILITY is DISABLED or FORCED at the database level (see above) this COMMIT option is ignored.
Syntax:
COMMIT [ { TRAN | TRANSACTION } ] [ transaction_name | @tran_name_variable ] ] [ WITH ( DELAYED_DURABILITY = { OFF | ON } ) ] References:
https://docs.microsoft.com/en-us/sql/relational-databases/logs/control-transaction-durability?view=sql-server-20