This function runs during save.  It runs for each item.  So, with core WPIM only, it runs after each reservation because only one item can be reserved at a time with just core.  If you have Reserve Cart, then this runs for each item in the cart.

function my_custom_function($inventory_id, $quantity, $original_quantity) {
   $inventory_id; // Item id
   $quantity; // Quantity reserved
   $original_quantity; // The quantity the item had at the time of the reserve (not including the reduction)

   // Get the item information

   $item_info = new WPIMItem();  // Call new instance of WPIMItem class
   $item_info = $item_info->get( $inventory_id );  // Get the item information with inventory id
   $name = $item_info->inventory_name;  // Example of how you would get the name of an item

    // Do whatever you want from here!

}

add_action('wpim_save_reserve', 'my_custom_function', 10, 3);