In SharePoint 2010 Designer workflow the User-Impersonation type step has some additional conditions available for checking list and item level permissions for a specified
Lets look at two major conditions that you would need to implement impersonation -

1. Check list item permissions - The condition simply says the below :
If permissions for these users are at least these permissions on item in this list.

The usage : If permissions for WFApprovers Members are at least Read on item in Current Items
For the above condition to evaluate as true, the users must have at least the permissions that the Read Permission levels provide. This step can very well be used to check that the WFApprovers should have read access to the items.

2. Check list item permission levels - The condition simply says the below :
If permission levels for these users are at least these permission levels on item in this list.
The usage : If permission levels for ikapoor Members are at least Read on item in Current Items.
Please note that : The users can be a single user, multiple users, but you cannot use a group in this condition.
Important Things to remember Notes : Impersonation steps can only be added to the root of a workflow and cannot be nested in another step.