what does a using statement without variable do when disposing?

From the specification:

8.13 The using statement

A using statement of the form

using (ResourceType resource = expression) statement

when ResourceType is a nullable value type or a reference type other than dynamic, the expansion is

{
    ResourceType resource = expression;
    try {
        statement;
    }
    finally {
        if (resource != null) ((IDisposable)resource).Dispose();
    }
}

A using statement of the form

using (expression) statement

has the same three possible expansions...The resource variable is inaccessible in, and invisible to, the embedded statement.

Therefore the object returned from cnctn.BeginTransaction() will be disposed when the block exits, but is not accessible inside the associated block.

From C# Specification 8.13 using statement defined as

using-statement:
   using (resource-acquisition) embedded-statement

Where resource-acquisition is

resource-acquisition:
    local-variable-declaration
    expression

In first case you have using which acquires resource via local variable declaration. In second case resource is acquired via expression. So, in second case resouce will be result of cnctn.BeginTransaction() call, which is DbTransaction from your DbProviderConnection class. Using statement disposes its resource after usage. So, yes, DbProviderConnection.Transaction.Dispose() will be called.

UPDATE: According to same article, your second using block will be translated to

DbTransaction resource = cnctn.BeginTransaction();
try
{
    //...
    cnctn.Transaction.Commit();
}
finally 
{
   if (resource != null) 
      ((IDisposable)resource).Dispose();
}