SET FORCEPLAN essentially overrides the logic used by the query optimizer to process a Transact-SQL SELECT statement. The data returned by the SELECT statement is the same regardless of this setting. The only difference is the way in which SQL Server processes the tables to satisfy the query.
When FORCEPLAN is set to ON, the SQL Server query optimizer processes a join in the same order as the tables appear in the FROM clause of a query. In addition, setting FORCEPLAN to ON forces the use of a nested loop join unless other types of joins are required to construct a plan for the query, or they are requested with join hints or query hints.